----------------
Changes
*******
- Added context manager return to lock to allow use of the 'with'
statement.
- Refactored to use zc.zk ZooKeeper library for higher level Zookeeper
abstraction with automatic watch re-establishment.
Features
********
- Node object to retrieve ZNode data from Zookeeper and keep it up
to date.
- Node objects can have data and children subscribers.
- NodeDict object that maps a shallow tree (one level of children)
into a dict-like object.
Backward Incompatibilities
**************************
- SharedZkLock has been refactored into ZkWriteLock and ZkReadLock.
- ``revoked`` is a property of Locks, not a method.
- ZkConnection is gone, lock objects, ZkNode, and ZkNodeDict all expect
zc.zk ZooKeeper instances.