Fix unit tests and put in a better fix for #80

* ZK tests needed to stop the client before deleting everything
  otherwise the ``_rebalance`` call from the watcher would re-create
  everything
* The fix for #80 is better off in the fetch code itself since
  an ``OffsetOutOfRange`` error can be caused by more than
  just zookeeper
* Include a unit test for the fix for #80