Added FacebookRequest::batch($session, $facebookRequestArray)

Code review comments and feedback requested.

example usage:

```
$req1 = new FacebookRequest(null, 'GET', '/me');
$req2 = new FacebookRequest(null, 'GET', '/me');

$responses = FacebookRequest::batch(
  $session,
  array($req1, $req2)
);

// $responses is now an array of FacebookResponse objects.
$me1 = $responses[0]->getGraphObject(GraphUser::className());
```