Implements the Sentinel API as mentioned on http://redis.io/topics/sentinel.
Sentinel is aware of master and slave nodes in a cluster and returns instances of Credis_Client accordingly.
The complexity of read/write splitting can also be abstract by calling the createCluster() method which returns a
Credis_Cluster object that contains both the master server and a random slave. Credis_Cluster takes care of the
read/write splitting