Re-bind closures to the application object on PHP 5.4

This allows using $app as $this in controllers and before/after handlers without having to use() it:

    $app->get('/', function () {
        return $this->redirect('/foo');
    });

Works for:

* closure controllers
* before/after filters
* error handlers
* finish handlers

It can also be disabled by overriding the `closure_rebinder` service, or via the `ControllerCollection::disableClosureRebinding()` method (particularly useful in controller providers if you want to access instance methods instead).