Add Kazoo 2.0 support as well as tests.

Kazoo 2.0 modified the KazooClient.add_auth() method to actually receive
and get the data for the asynchronous add authentication call. This
deadlocks when its wrapped inside of a KazooClient.retry() method.

The fix is to call add_auth_async() directly and ignore the result.
Additionally, I've thrown in tests that triggered this behavior, and
added Kazoo 2.0 to the testing matrix for Travis.
Add changelog and make the tests more verbose.
Merge pull request #56 from Nextdoor/issue_48

Add Kazoo 2.0 support as well as tests.