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: 
Perhaps rename to ```initializeSubjectInBackend``` and change its contract to doing the necessary work for a Subject to exist in the backend.

Alternatively, merge ```registerInternal``` and ```createSubjectInternal``` and combine their contracts as well. In which case the merged function could be called ```createSubjectInBackend```.

I think explicitly specifying that an operation is intended to be performed against a backend helps understand the goal of the API more easily...

Let me know what you think about the naming suggestions...!