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 think the fact that most (or all?) Repository implementations of this function currently return null confirms my hunch that this probably belongs in just one place (the REST Server) and not in the Repo interface...

Of course we could manage to make it return something else than null, but it seems like it might have had a more self-evident return value if it was meant to be there in the first place.