Allow setting of Session FlashBag and AttributeBag.

This change allows you to set the FlashBag and AttributeBag for the Symfony Session when registering the SessionServiceProvier:

    $app->register(new Silex\Provider\SessionServiceProvider(), array(
        'session.storage.options' => array(
          'name' => 'mycookie'
        ),
        'session.flashbag' => new Symfony\Component\HttpFoundation\Session\Flash\AutoExpireFlashBag
      )
    );

Allows options equivalent to Symfony's `session.flashbag.class` and `session.attribute_bag.class` options.