A major feature release with a significant documentation overhaul and long overdue API cleanup!

This update involves a number of BC breaks due to dropped support for deprecated
functionality. We've tried hard to avoid BC breaks where possible and minimize
impact otherwise. We expect that most consumers of this package will actually
not be affected by any BC breaks, see below for more details.

We realize that the changes listed below may seem overwhelming, but we've tried
to be very clear about any possible BC breaks. Don't worry: In fact, all ReactPHP
components are already compatible and support both this new release as well as
providing backwards compatibility with the last release.

• Feature / BC break: Add support for signal handling via new
  LoopInterface::addSignal() and LoopInterface::removeSignal() methods.
  (#104 by @WyriHaximus and #111 and #150 by @clue)

  $loop->addSignal(SIGINT, function () {
      echo 'CTRL-C';
  });

• Feature: Significant documentation updates for LoopInterface and Factory.
  (#100, #119, #126, #127, #159 and #160 by @clue, #113 by @WyriHaximus and #81 and #91 by @jsor)

• Feature: Add examples to ease getting started
  (#99, #100 and #125 by @clue, #59 by @WyriHaximus and #143 by @jsor)

• Feature: Documentation for advanced timer concepts, such as monotonic time source vs wall-clock time
  and high precision timers with millisecond accuracy or below.
  (#130 and #157 by @clue)

• Feature: Documentation for advanced stream concepts, such as edge-triggered event listeners
  and stream buffers and allow throwing Exception if stream resource is not supported.
  (#129 and #158 by @clue)

• Feature: Throw BadMethodCallException on manual loop creation when required extension isn't installed.
  (#153 by @WyriHaximus)

• Feature / BC break: First class support for legacy PHP 5.3 through PHP 7.2 and HHVM
  and remove all callable type hints for consistency reasons.
  (#141 and #151 by @clue)

• BC break: Documentation for timer API and clean up unneeded timer API.
  (#102 by @clue)

  Remove TimerInterface::cancel(), use LoopInterface::cancelTimer() instead:

  // old (method invoked on timer instance)
  $timer->cancel();
  
  // already supported before: invoke method on loop instance
  $loop->cancelTimer($timer);

  Remove unneeded TimerInterface::setData() and TimerInterface::getData(),
  use closure binding to add arbitrary data to timer instead:

  // old (limited setData() and getData() only allows single variable)
  $name = 'Tester';
  $timer = $loop->addTimer(1.0, function ($timer) {
      echo 'Hello ' . $timer->getData() . PHP_EOL;
  });
  $timer->setData($name);
  
  // already supported before: closure binding allows any number of variables
  $name = 'Tester';
  $loop->addTimer(1.0, function () use ($name) {
      echo 'Hello ' . $name . PHP_EOL;
  });

  Remove unneeded TimerInterface::getLoop(), use closure binding instead:

  // old (getLoop() called on timer instance)
  $loop->addTimer(0.1, function ($timer) {
      $timer->getLoop()->stop();
  });
  
  // already supported before: use closure binding as usual
  $loop->addTimer(0.1, function () use ($loop) {
      $loop->stop();
  });

• BC break: Remove unneeded LoopInterface::isTimerActive() and
  TimerInterface::isActive() to reduce API surface.
  (#133 by @clue)

  // old (method on timer instance or on loop instance)
  $timer->isActive();
  $loop->isTimerActive($timer);

• BC break: Move TimerInterface one level up to React\EventLoop\TimerInterface.
  (#138 by @WyriHaximus)

  // old (notice obsolete "Timer" namespace)
  assert($timer instanceof React\EventLoop\Timer\TimerInterface);
  
  // new
  assert($timer instanceof React\EventLoop\TimerInterface);

• BC break: Remove unneeded LoopInterface::nextTick() (and internal NextTickQueue),
  use LoopInterface::futureTick() instead.
  (#30 by @clue)

  // old (removed)
  $loop->nextTick(function () {
      echo 'tick';
  });
  
  // already supported before
  $loop->futureTick(function () {
      echo 'tick';
  });

• BC break: Remove unneeded $loop argument for LoopInterface::futureTick()
  (and fix internal cyclic dependency).
  (#103 by @clue)

  // old ($loop gets passed by default)
  $loop->futureTick(function ($loop) {
      $loop->stop();
  });
  
  // already supported before: use closure binding as usual
  $loop->futureTick(function () use ($loop) {
      $loop->stop();
  });

• BC break: Remove unneeded LoopInterface::tick().
  (#72 by @jsor)

  // old (removed)
  $loop->tick();
  
  // suggested work around for testing purposes only
  $loop->futureTick(function () use ($loop) {
      $loop->stop();
  });

• BC break: Documentation for advanced stream API and clean up unneeded stream API.
  (#110 by @clue)

  Remove unneeded $loop argument for LoopInterface::addReadStream()
  and LoopInterface::addWriteStream(), use closure binding instead:

  // old ($loop gets passed by default)
  $loop->addReadStream($stream, function ($stream, $loop) {
      $loop->removeReadStream($stream);
  });
  
  // already supported before: use closure binding as usual
  $loop->addReadStream($stream, function ($stream) use ($loop) {
      $loop->removeReadStream($stream);
  });

• BC break: Remove unneeded LoopInterface::removeStream() method,
  use LoopInterface::removeReadStream() and LoopInterface::removeWriteStream() instead.
  (#118 by @clue)

  // old
  $loop->removeStream($stream);
  
  // already supported before
  $loop->removeReadStream($stream);
  $loop->removeWriteStream($stream);

• BC break: Rename LibEventLoop to ExtLibeventLoop and LibEvLoop to ExtLibevLoop
  for consistent naming for event loop implementations.
  (#128 by @clue)

• BC break: Remove optional EventBaseConfig argument from ExtEventLoop
  and make its FEATURE_FDS enabled by default.
  (#156 by @WyriHaximus)

• BC break: Mark all classes as final to discourage inheritance.
  (#131 by @clue)

• Fix: Fix ExtEventLoop to keep track of stream resources (refcount)
  (#123 by @clue)

• Fix: Ensure large timer interval does not overflow on 32bit systems
  (#132 by @clue)

• Fix: Fix separately removing readable and writable side of stream when closing
  (#139 by @clue)

• Fix: Properly clean up event watchers for ext-event and ext-libev
  (#149 by @clue)

• Fix: Minor code cleanup and remove unneeded references
  (#145 by @seregazhuk)

• Fix: Discourage outdated ext-libevent on PHP 7
  (#62 by @cboden)

• Improve test suite by adding forward compatibility with PHPUnit 6 and PHPUnit 5,
  lock Travis distro so new defaults will not break the build,
  improve test suite to be less fragile and increase test timeouts,
  test against PHP 7.2 and reduce fwrite() call length to one chunk.
  (#106 and #144 by @clue, #120 and #124 by @carusogabriel, #147 by nawarian and #92 by @kelunik)

• A number of changes were originally planned for this release but have been backported
  to the last v0.4.3 already: #74, #76, #79, #81 (refs #65, #66, #67), #88 and #93