Introducing a wrapper to the Guzzle oAuth plugin

I was toying around with the React HttpClient, and found that it lacked the support for oAuth.

I would like to hear your thoughts and feedback on this.

Some example code:

```php
<?php

require_once 'vendor/autoload.php';

$loop = React\EventLoop\Factory::create();

$resolverFactory = new React\Dns\Resolver\Factory();
$httpClientFactory = new React\HttpClient\Factory();

$dnsResolver = $resolverFactory->createCached('8.8.8.8', $loop);
$client = $httpClientFactory->create($loop, $dnsResolver);

$oauth = new React\HttpClient\Authentication\Oauth(array(
    'consumer_key' => '...',
    'consumer_secret' => '...',
    'token' => '...',
    'token_secret' => '...'
));

$request = $client->request('GET', 'https://stream.twitter.com/1.1/statuses/sample.json');
$request->on('response', function($response) {
    $response->on('data', function($data) {
        echo $data;
    });
});
$request->on('writing-headers', array($oauth, 'onWritingHeaders'));
$request->end();
$loop->run();

```