To implement issue informatikr#49, add a new multithreaded Pub/Sub message processing feature.
The main benefit over the existing Pub/Sub code (which is left unchanged) is:

- you can make subscription changes at any time
- you can safely recover from networking errors such as the redis servier dying
- you can detect when Redis has actually processed a subscription request and
  handlers will now start receiving messages.

- Minor fixes suggested by reviewers on github
- Added ability to unsubscribe from just the handlers
  you register, instead of unregistering all handlers
  for a given channel or pattern channel name

This tests almost everything.  The only thing missing from the
test is proper reconnection after the redis server network
connection dies, but that can't be automated.