Add initial support for Redis Pub/Sub.
Merge remote-tracking branch 'upstream/master' into redis-pubsub
Revise Redis Pub/Sub method signatures and implement the callback mechanism.
Redis Publish takes string arguments only.
Move Redis subscription listening code to RedisSubscriber.
Make RedisSubscriber.listen() asynchronous.
Add Redis Pub/Sub async subscribe/unsubscribe support.
Merge pull request #515 from metafex/redis-pubsub

Add initial support for Redis Pub/Sub.