Http client adapters

This is a draft implementation of HTTP adapters solving #49. 

The library now supports:
* Guzzle 3
* Guzzle 4
* Buzz

The user has to choose an adapter when creating the API client :

```php
<?php

$buzz = new \Gitlab\HttpClient\Adapter\BuzzAdapter();
$guzzle3 = new \Gitlab\HttpClient\Adapter\Guzzle3Adapter();
$guzzle4 = new \Gitlab\HttpClient\Adapter\Guzzle4Adapter();

$bgitlab = new \Gitlab\Client('http://git.foobar.com/api/v3/', $buzz);
$bgitlab->authenticate('foobar', \Gitlab\HttpClient\HttpClientInterface::AUTH_HTTP_TOKEN);

$g3gitlab = new \Gitlab\Client('http://git.foobar.com/api/v3/', $guzzle3);
$g3gitlab->authenticate('foobar', \Gitlab\HttpClient\HttpClientInterface::AUTH_HTTP_TOKEN);

$g4gitlab = new \Gitlab\Client('http://git.foobar.com/api/v3/', $guzzle4);
$g4gitlab->authenticate('foobar', \Gitlab\HttpClient\HttpClientInterface::AUTH_HTTP_TOKEN);
```

:warning: This will break user's code when updating the library :warning: but I think it's a minor change.
cc @sstok