I hate to be that guy, but this already exists as a stand-alone iOS app http://www.imore.com/iphone-proximity-sensor-locks-mac-keycard-mac http://www.imore.com/

I hate to be that guy, but this already exists as a stand-alone iOS app http://www.imore.com/iphone-proximity-sensor-locks-mac-keycard-mac http://www.imore.com/iphone-proximity-sensor-locks-mac-keyca...