Zookeeper driver support

Adds a zookeeper driver that matches conventions used in the etcd, memcached, and redis libraries.  All tests pass.

Note that unit tests require zookeeper server to be started by hand on :2182 so this may not be able to be merged until it can be modified to be skipped.

Also, I don't yet clean up dead backends after the ttls have expired, I just check the ttl on read.  A future iteration should remove znodes in the dead path.