Clarify WS4REDIS_SUBSCRIBER
Clarify WS4REDIS_SUBSCRIBER

`ws4redis.store.RedisStore` doesn't exist in 0.5.1 as the module has been moved to `redis_store.py`.
Took the chance to rewrite the settings document to clarify the default behavior and how to extend the base class.

jrief/django-websocket-redis,jrief/django-websocket-redis