Issue 15 + Refactoring

I've made fairly significant changes to the class hierarchy and I'm in doubts whether they are to the better.
Please ignore the status and config part for the time being and let me know if the *Repository changes make sense. Specifically, I tried to extract common subject registration/lookup logic into AbstractSubjectValidatingRepository and succeeded for for InMemory and LocalFileSystem but with Zookeeper I was afraid to significantly alter the flow of the code and finished up using the new base class in a somewhat awkward manner. The tests pass, but I don't feel terribly good - so any comments, whether positive or negative will help :smile: 
I do like having a common suffix as well, but I find ```Internal``` a bit vague.

The way I see it, ```AbstractBackendRepository``` is the common code for wrapping some backend storage system (currently, those "backends" can be a HashMap, the local filesystem or ZK).

This is why I suggested ```InBackend``` as a common suffix. Except for ```createSubjectInternal``` where I suggested ```instantiateSubject``` since it seems to be mostly just doing a constructor call, rather interacting with the backend directly.