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 build and 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 will be fixed when we add redis-cluster support.

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