Add new features about Sentinel runtime configuration API introduced on Redis 2.8.4

Redis 2.8.4 introduces new features : Sentinel runtime configuration API
(Details are documented here : http://redis.io/topics/sentinel)

I have implemented and test it, and it works so I think we can merge to master.
(I have tested with Redis unstable branch, most recent version : 80e80668f47698320bff4fe45ce2ecd2a463d787)

* Implements new sentinel commands (failover, monitor, remove, set) 
* unit test included
  * added 2 redis-server and 1 sentinel for failover test
* with some refactoring
  * SentinelCommands : refactor to have interface
  * HostAndPortUtil : same format to cluster setup