Cleanup and merge of backend from Disqus.
Cleanup and merge of backend from Disqus.

* Respect typical Django conventions by checking if the key exists and returning CreateError if must_create == True.
* Changing expiration date to an absolute point in time
* Only return data on load() if the session hasn't expired
* Remove seemingly unccessary self.riak_obj

flashingpumpkin/django-riak-sessions