Two hooks for DoctrineBundle to allow ContainerFilterCollection

Allows the DoctrineBundle to inject a ```ContainerFilterCollection``` that creates filters through dependency injection. See doctrine/DoctrineBundle#245.

Would prefer to inject the ```FilterCollection``` into the constructor of the em, but since they have a cyclic dependency I chose setter injection.
Where should I move it / how should I autoload it?

This is according to an example found at FilterCollectionTest, btw :-).