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 ConnectionFeactory for HA environments. 

**Using Jedis**
Providing RedisSentinelConfiguration will force the JedisConnectionFactory to use JedisSentinelPool for managing resources.

**Using Lettuce/JRedis/SRP**
There’s currently no support for sentinel in those clients.

**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. This is somehting we need to get fixed when adding cluster support.

_side note:_ there’s an alternative fork of lettuce at mp911de/lettuce that already has sentinel support.