Add embedded runit to Sensu packages

As an administrator deploying Sensu, I would like Sensu to ship with embedded runit so that I can reasonably expect processes like sensu-server to automatically restart when an unhandled exception is encountered.

As a maintainer packaging Sensu, I would like Sensu to use embedded runit as the "official" means of starting and supervising Sensu processes so that I can stop maintaining many different flavors of init scripts.