Support for redis sentinel in redis input and output

Redis supports failover through a separate service called Sentinel (http://redis.io/topics/sentinel). I added support for it to the output and input modules using the redis-sentinel wrapper library (https://github.com/flyerhzm/redis-sentinel).
Yup, ```sentinel_master``` is a good idea. I suppose it would make sense to require it when connecting to sentinels. I can fix these once we reach a conclusion on the host/port point above.