(TWILL-74) Added authentication and ACL support to ZKClient.

* New method, addAuthInfo, added to ZkClientService.Builder for creating ZKClient with authentication.
* New methods (create, getACL, setACL) to support ACL are added to ZKClient.
* Refactor ZKClient hierarchy to have a AbstractZKClient to make sure correct behavior of method delegation for children classes.

Signed-off-by: Terence Yim <terence@continuuity.com>