Added transport to support egeloen/http-adapter

Supporting [egeloen/http-adapter](https://github.com/egeloen/ivory-http-adapter) will
allow to make http calls using  multiple libraries (including Guzzle3) :

* Buzz
* Cake
* cURL
* FileGetContents
* Fopen
* Guzzle
* GuzzleHttp
* Httpful
* React
* Socket
* Zend1
* Zend2

Here is an example of how to use it :

```php
  $guzzle3 = new \Guzzle\Http\Client();

  $httpAdapter = new \Ivory\HttpAdapter\GuzzleHttpAdapter();
  $httpAdapterTransport = new \Elastica\Transport\HttpAdapter(null, $httpAdapter);

  $elastica = new \Elastica\Client(array(
      'connections' => array(
          array(
          '   host' => 'localhost',
              'port' => 9200,
              'transport' => $httpAdapterTransport,
          )
      )
  ));

  var_dump($elastica->getIndex('livre')->getType('livre')->getDocument('9782212136777')->getData());
```
Please use _getUri for the protected methods.