Migration: bundle system.

This pull requests provides a proper way to load existing Symfony bundles.

I've started implementing this to integrate Monolog. Since there is already a proper Monolog Bundle available, it would be stupid to create a new bundle, specific for Fork.

We can now properly integrate existing bundles like Monolog and Swiftmailer to rapidly integrate these features.

One note though, we need to be sure the dependencies match ours.