Added simple launchd script
Assumes shairport binary at /usr/bin/shairport
From http://launchd.info/:
launchd differentiates between agents and daemons. The main difference is that an agent is run on behalf of the logged in user while a daemon runs on behalf of the root user or any user you specify with the User key.
User Agents     ~/Library/LaunchAgents  Currently logged in user
Global Agents   /Library/LaunchAgents   Currently logged in user
Global Daemons  /Library/LaunchDaemons  root or the user specified with the key User