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've refactored LocalFileSystemRepository based on your comment, you were correct (in terms of `createSubjectInternal` vs `registerInternal`). I hope the intent is clear from the javadocs: the former is responsible for creating the actual subject instance without any durable side-effects, so there is no harm in calling it more than once for a given subject, the latter does the actual heavy lifting and is called only to materialize the persistent part.