[ADD] redis-sentinel support

Hello,

first I need to say: "I love your cookbook for redis" :+1: 

but I am using redis-sentinel for failover so I tried to implement starting and configuring redis sentinel by your cookbook.

This pull request contains changes for adding support of sentinel.

see http://redis.io/topics/sentinel

Sorry for my english and I hope I don;t have any error in it. But I tested it in vagrant precise64.box and it worked flawlessly

Ondrej Bartas