* better separation of concerns
* made TwigBundle independant of the PHP Engine from FrameworkBundle (WIP)
* removed one layer of abstraction in the Templating component (renderers)
* made it easier to create a new Engine for any templating library
* made engines lazy-loaded (PHP engine for instance is not started if you only use Twig)
* reduces memory footprint (if you only use one engine)
* reduces size of compiled classes.php cache file
* made the renderer argument of Storage ctor mandatory
* refactored the Engine class to avoid code duplication
* simplified the check for a template that extends another one but with a different renderer
