Schema support - path validation and documentation

ZooKeeper applications are heavily dependent on correct usage of paths, watchers, ZNode data, etc. Currently, there is no mechanism for validating and documenting this. It would be nice to have some kind of schema system that allows for this.

This issue provides several items of functionality: a) ZNode path documentation; b) ZNode path validation; c) keyed reference to ZNode paths. Both items are defined in a new class, Schema. A Curator Schema specifies a ZNode path (or regex pattern), documentation for that path, and the operations that are allowed on that path. Additionally, a set of Schema objects are collected into a SchemaSet object that allows any Schema (and thereby its path) to be retrieved via a symbolic name. This should make path management in ZK applications easier. Lastly, a utility is provided to load SchemaSets from a file/stream in JSON format.