Add support for Redis Sentinel Configuration. 

Spring Data Redis 1.4.0 will introduce redis [Sentinel](http://redis.io/topics/sentinel) support. Sentinels can be configured using `RedisSentinelConfiguration`. When applied to `RedisConnectionFactory` the sentinel configuration will be used to determine current master node an perform failover in case a new master is elected.

Added new config properties to `RedisProperties`:

```
spring.redis.sentinel.master=mymaster #name of redis server
spring.redis.sentinel.nodes=127.0.0.1:26379,127.0.0.1:26380 #deliminated list of sentinels.
```

In case we detect a bean of type `RedisSentinelConfiguration` we’ll use that one to configure the `RedisConnectionFactory`.

In order to get the tests working in a non monitored environment we added a check for the availability of sentinel nodes.


For some reason there've been some issue building the project - that's why there are 2 commits here - please skip the first one. 

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**Note:** At this time sentinel support is only available for [jedis](https://github.com/xetorthio/jedis/).