Sentinel

Hi, I have added initial support for redis sentinel failover. 

I decide to add it to eredis instead of standalone lib, what do you think about it?
For now sentinel is experimental feature but it should be main aproach to monitor redis clusters and many already use it in production.
 
Also I see some issues with current implementation of eredis:

1. Shouldn't we have to check Socket when processing {tcp, XXX} messages and ignore data from already closed socket. With sentinel we can start reconnecting to new master in the middle of processing reply for example.

2. When cleaning queue of waiting clients eredis don't send them any notification, maybe it will be better to send them error reply in this case. 
  
Thanks.