PubSub Unsubscribe Issues Patch

Hey Andy,

This should fix [issue #254](https://github.com/andymccurdy/redis-py/issues/254) and  [issue #255](https://github.com/andymccurdy/redis-py/issues/255).

I am not sure if it is in line with your design, but it did work out for me. 

Hope this helps!