Add in launchd script for OS X.

Set up a daemon service for OS X using launchd that loads on boot.
The script depends on the computer having a network connection and
upon the mDNSResponder and coreaudiod running before it launches
shairport.

Other thoughts: Looking through the launchd options it would seem
that you could advertise the service using Bonjour via launchd
itself and then make it run only when a connection hits the port. I
think shairport would need to be changed so that it doesn't try and
advertise the service.