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: 
Do you like this pattern? Passing a param and mutating it with a void return type?

I understand that it works, but it seems a bit messy to me... Granted, this is not something people would need to override all the time, so we can control that it's used correctly when getting PRs... but I'd rather have a less error-prone API...