Listen on Sentinel PUBSUB channel for +switch-master
Fix the Pub/Sub implementation to fix unit tests and not have a singleton.

Problem:
    The original design of the sentinel with Pub/Sub to the switch-master
    message broke the last unit test because it was always connected to the
    previously successful servers.

Analysis:
    In order to not have always stay connected to the old servers the
    subscription to the Pub/Sub message needs to only occur when the
    successful connect occurs.
Fix race condition in refresh by not connecting until resolver is complete.

The resolver needs to run before the place to connect is determined, but
node-redis will start attempting to reconnect immediately. Prevent this by
setting the port and ip to the empty string and letting the resolver
modify it.

A better fix would be to modify node-redis to be able to not retry until
told to do so.
Merge pull request #25 from ldm5180/pubsub-switchmaster

Pub/Sub subscribe to +switch-master events.