SUBSCRIBE/PSUBSCRIBE

Implemented SUBSCRIBE and PSUBSCRIBE for Redis Cluster.  Currently
Cluster will send publish messages to every node, so we can subscribe
wherever we like.
UNSUBSCRIBE/PUNSUBSCRIBE

Implemented the two unsubscribe commands in Redis Cluster.
Presently, like with the standard Redis class once you subscribe
you're there for good, but it would be nice to be able to use the
callback return value to break out of the subscribe loop.