Add ethernet-based wifi on/off script.

Source:
http://www.computernetworkbasics.com/2012/12/automatically-turn-off-wireless-in-osx-including-mountain-lion/
via @xavez

Script turns wireless network connection on or off, based on whether or
not the ethernet cable is plugged in.  It needs some plist loving in
LaunchDaemons (see comments)
Installation/setup should be automated in
https://github.com/dubgeiser/mbinstall