Adding Guzzle Client

Clean-up of my previous pull request #155.

Basic implementation of Guzzle HTTP client. Includes the ability to catch Guzzle Exceptions for non-2XX HTTP status codes, and thus read the HTTP body for errors and other important information

Sample Usage:

    $serviceFactory = new \OAuth\ServiceFactory();
    $guzzleClient = new OAuth\Common\Http\Client\GuzzleClient;
    $serviceFactory->setHttpClient($guzzleClient);
    
    $slapi = $serviceFactory->createService('SellerLabs', $credentials, $storage);
    
    try {
        $response = $slapi->request('/foo', 'GET');
    } catch (OAuth\Common\Http\Exception\GuzzleException $e) {
        $response = $e->getGuzzleResponse();
        $http_status = $response->getStatusCode();
        echo "<h4>SLAPI Request returned a HTTP {$http_status} error</h4>\n"; 
        $error = $response->getBody();
        echo "Raw Response:\n<pre>". $error. "</pre>\n";
    }