Add launchctl plist

To be symlinked to from /System/Library/LaunchDaemons in order to run
serve.sh on system startup. Be sure that permissions are set to
root:wheel in order for launchctl to load and run it at boot.

See:
 - http://nathangrigg.net/2012/07/schedule-jobs-using-launchd/
 - https://discussions.apple.com/thread/2749274?start=0&tstart=0
 - https://developer.apple.com/library/mac/documentation/MacOSX/Conceptual/BPSystemStartup/Chapters/CreatingLaunchdJobs.html
 - https://developer.apple.com/library/mac/technotes/tn2083/_index.html#//apple_ref/doc/uid/DTS10003794
 - https://developer.apple.com/library/mac/documentation/Darwin/Reference/ManPages/man5/launchd.plist.5.html#//apple_ref/doc/man/5/launchd.plist
Add debug logs to launch daemon config

Also, fully-qualify the path to git as it was otherwise not being found
when running as root.

See "Debugging launchd jobs" at https://developer.apple.com/library/mac/documentation/MacOSX/Conceptual/BPSystemStartup/Chapters/CreatingLaunchdJobs.html#//apple_ref/doc/uid/10000172i-SW7-BCIEDDBJ
Update serve.sh to survive force push / rebase