Added an implementation of Sentinel Support to Redis

This is an initial implementation of a Redis Sentinel that will monitor a set of Sentinels and manage an instance of a PooledRedisClientManager with the latest/greatest set of master and slaves.

To use it, do the following:
    var sentinel = new RedisSentinel(new string[] {"sentinel1:port", "sentinel2:port", "etc"}, "sentinel name");
    var clientManager = sentinel.Setup();
    container.Register<IRedisClientsManager>(clientManager);