hook the DatabaseLoader into the Translator service via a compiler pass

This is a proposal to solve issue #6. The drawback of this solution is that you have to configure the locales and message domains handled by the ``DatabaseLoader`` in your app configuration.

Another possible solution I can think of would be to extend the ``Translator`` service and use the ``DatabaseLoader`` as some kind of fallback translator. Don't know if I like that better. Seems a bit hacky.