delete now returns number of deleted keys instead of None

It might be usefull to allow app to know whether the key was actually deleted
or it was not in the DB. Redis does this by returning the number of keys deleted.
django-redis could mimic that behaviour without
breaking any existing functionality.
tests: test_timeout_negative: also test that key can be expired without being in the db.
Merge pull request #62 from teferi/master

.delete returning number of deleted items instead of None.