Add a cached translator

TranslationServiceProvider provides the use of symfony/translation component.
This component is nice but it's quite slow as loaders parse the translations sources on every requests.

I've added a `Silex\CachedTranslator` based on Symfony Framework bundle [one](https://github.com/symfony/symfony/blob/master/src/Symfony/Bundle/FrameworkBundle/Translation/Translator.php).