Issue #17: Replace time.sleep() with client.handler.sleep_func()

This puts more reliance on Kazoo into the code, but means that if we
switch to Gevent at any point (or provide the user with that option),
our sleep timer will become a gevent sleep timer properly and
will not cause blocking issues.

Change-Id: Ic5e4a3af152ceebe35d7e0d807dee6c3a088b6d4