Add functionality to send batched requests to graph API

[gary-rafferty]

When making multiple calls to the graph API, it is often handy to batch up the requests and make a single call.
This commit allows you to make calls like

$facebook = new Facebook(array(
  'appId'  => APP_ID,
  'secret' => SECRET
));

$batch_data = array(
  array('method' => 'GET', 'relative_url' => 4),
  array('method' => 'GET', 'relative_url' => 5)
);

$response = $facebook->batch($batch_data);

// Verify the response(s)
$this->assertEquals(2, count($response));
$this->assertEquals(4, $response[0]['id']);
$this->assertEquals(5, $response[1]['id']);
$this->assertEquals('zuck', $response[0]['username']);
$this->assertEquals('ChrisHughes', $response[1]['username']);

[gfosco]

Hi Gary,

This sounds like a good addition to the library. Once you've signed the CLA (I commented on one of your other PRs with the URL,) we can further review and consider this.

Thanks for your contribution,

Fosco

[gary-rafferty]

Hi Fosco,

The CLA has now been signed. Thanks,

[gfosco]

We'll look to add batch support to a future release, after the main codebase is refactored and namespaced with newer PHP features. We also want to spend some time designing the API and fully test it. It will be tracked by issue #109

Thanks for your contribution.