Implemented propel storage backend

This is a complete propel storage implementation including tests (at least the same tests that are already in place for the orm/mongodb storage).

To make things work, I had to make a few changes to the existing classes and interfaces (mainly replacing class references by new interfaces), so technically it might be a BC break for people who have overridden some of the services. For users just following the installation docs everything should be fine.

Besides the unit tests (everything green) I did some manual tests on the translation frontend and it seems to work great. I can't test the doctrine/mongodb version here - someone else should do this to make sure everything is ok.

I hope this gets merged - I've been looking for a good propel based translator for a while and patching this one was actually the easiest solution.