Add functionality to send batched requests to graph API

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

```php
$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']);

```