This is a follow-up to #223.

Greetings!

The changes in this PR remove Guzzle as a dependency and allows developer to use their already existing HTTP client as long as it is supported by the Ivory HTTP adapter (which, as a bonus point, implements PSR-7).

In the default state, the CurlHttpAdapter is used (which only has curl as a dependency), but if a project is already using another HTTP client, it can be configured like this:

use Ivory\HttpAdapter\GuzzleHttpHttpAdapter;

$client = new \GuzzleHttp\Client(); // or any existing client
$httpClient = new GuzzleHttpHttpAdapter($client); // Guzzle 5

$provider = new League\OAuth2\Client\Provider\<ProviderName>([
    'clientId'      => 'XXXXXXXX',
    'clientSecret'  => 'XXXXXXXX',
    'redirectUri'   => 'https://your-registered-redirect-uri/',
    'scopes'        => ['email', '...', '...'],
], $httpClient);

You can find a list of supported adapters and a full usage documentation in the Ivory HTTP Adapter README.

I hope this PR receives your approval, and your feedback would be highly appreciated.

Cheers
Jérôme