Make FakeStrictRedis inherit redis.StrictRedis

Some libraries check whether the type of an argument is
redis.StrictRedis or not. If FakeStrictRedis is complete, this
inheritance should make no difference.
Make FakeRedis inherit redis.Redis
Support Python 3.3

Closes #23
Implement pttl
Merge branch 'inherit-redis'
Merge branch 'py33'
Merge branch 'pttl'