DATAREDIS-324 - Add Redis Sentinel support. 

We’ve added `RedisSentinelConfiguration` holding required information for connecting to Redis Sentinels. This can be used to set up `ConnectionFactory` for HA environments. 

**Using Jedis**
Providing `RedisSentinelConfiguration` will force `JedisConnectionFactory` to use `JedisSentinelPool` for managing resources.

**Using Lettuce / JRedis / SRP**
There’s currently **no support** for Sentinel in those clients.
_Side Note:_ there’s an alternative fork of lettuce at [mp911de/lettuce](http://github.com/mp911de/lettuce) that already has sentinel support.

**CI Build**
We’ve added makefile to set up redis instances for testing sentinel support on travis-ci. There’s already a section for redis cluster. The cluster section is for whatever reason currently not working as the cluster nodes won’t start - so it is disabled right now. This is somehting we need to get fixed when adding cluster support.