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: 
Yes, this is my most serious omission. I didn't realize ZooKeeperSubject constructor is not lightweight. Overall, the way logic split between XYZRepository and XYZSubject seems somewhat inconsistent. This will need to be addressed at some point. So you suggest that ZookeeperRepository simply overrides `subject()` itself, and the default (AbstractBackendRepository) impl doesn't do anything fancy (which will still satisfy LocalFS and InMemory). Right?