Sentinel support for Redis-rb

Hello, this PR adds Sentinel support to redis-rb, following the new clients - sentinels [handshake protocol](http://redis.io/topics/sentinel-clients). Additional info:

* The code was tested by hand but no tests are provided with this commits, since I've not enough expertise with the Ruby testing ecosystem to write a few tests in a reasonable amount of time.
* There is an hard-coded 300 milliseconds timeout to talk with Sentinels. This should be appropriate in most contests, but maybe to make it configurable is a good idea.

Regards,
Salvatore