Adding Sentinel command to RedisNativeClient

I added a single Sentinel command to the RedisNativeClient to support sending sentinel specific messages to redis servers running in sentinel mode.  The command takes two arguments: a command name and an optional master name.  All of the documented sentinel commands follow this format, so I figured it should be sufficient (as opposed to a params or something..)

I also wrote a couple of simple tests to make sure the various commands worked at least generally (we got data back, etc.) and tried to match it up as well as I could with the existing test config.