Dropwizard Extra ZooKeeper

Added a new module that introduces ZooKeeper integration in Dropwizard.
Changed ZooKeeper initialization to composition

ManagedZooKeeperClient no longer inherits from ZooKeeper class. Instead it only manages an instance. The ZooKeeper instance is created by the ZooKeeperClientFactory class
Fixed javadoc for ManagedZookeeperClient
Changed ZookeeperClientFactory to ZooKeeperClientFactory

That's with a capital K to be inline with the underlying library
Merge pull request #4 from datasift/dropwizard-zookeeper

Dropwizard Extra ZooKeeper

New module providing lifecycle management for the ZooKeeper client library.