add runit service module

This is a module for the [runit](http://smarden.org/runit/) service supervisor.

Since runit adopted many conventions from daemontools, this module was based on the daemontools module already available in salt. Like the daemontools module, it can function as a provider for the service state, but it is assumed that it will not be used as a primary service provider.

Please let me know if it needs any changes!