Skeleton of the ZK backend implementation.
Lots of stuff remaining to do.
Changed version to 0.1.2-SNAPSHOT
ZooKeeperRepository starts up, gets configured, and creates ZK namespace.
Also refactored some config code.
Added a config section to the README

Including information on how to configure validators, the file-based and ZooKeeper-based backends and Jetty.
Refactored configs some more so that the LocalFileSystem Repo is consistent with the rest.
ZooKeeperRepository passes the general unit tests.
There are still some rought edges, so the implementation is not as efficient as it could be. I have left TODOs in the code to address those. We should also add automated tests specifically for the high-availability.