Add support for Redis servers that require authentication

Since Redis Sentinel doesn't currently support authentication, you cannot just include `:password`, as that will get passed along when the sentinel connects triggering a `ERR unknown command 'auth'` error.

All this does is add a new option `:master_password` that will only be passed to the Redis server it connects to that it gets back from Sentinel. This also means if/when Sentinel supports authentication, you can use a different password for the sentinel vs the redis master.