Accept redis & subscriber client as options

Wondering if we could accept options for the `redis` & `subscriber` objects.

It'd be nice if we can pass in a custom redis client (any `node_redis` compatible client). I'm trying to use [node-redis-sentinel](https://github.com/ortoo/node-redis-sentinel) which is a `node_redis` compatible client that implements the [redis sentinel connection routine](http://redis.io/topics/sentinel) for a high availability redis setup.

The submitted code is just to get the discussion started, it could probably use some tweaks.