Server::listen() is blocking if given a hostname

> Migrated from https://github.com/reactphp/react/issues/181, originally reported 2013-04-13

Calling Socket\Server::listen($port, $host) with a hostname instead of an IPv4 address is a blocking operation.

Example:

``` php
$server = new Socket\Server($loop);
$server->listen(1234, 'example.com');
```

Giving a hostname will result in resolving it via DNS (blocking).
