DATAREDIS-324 - Add Redis Sentinel support.

We’ve added RedisSentinelConfiguration holding required information for connecting to redis sentinels. This can 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.

**Open Issues**
* DONE: Setup Junit rule checking if we’ve sentinels running and skip test if not.
* DONE: Integration tests.
* TODO: Set up travis-ci to use sentinels.
* TODO: Update documentation.


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