Adds a new class, ZKSchema, to handle schema management in Zookeeper in
an automated and consistent way. This should solve several issues:
1. Pain in managing changes to ZK keys
2. Pain in handling those changes during live upgrades
3. Simplifying the codebase to remove hardcoded ZK paths
The current master schema for PVC 0.9.19 is committed as version 0.
Addresses #129
