Openbeam Optical Limit Switch Mount

Allows mounting an optical limit switch module on 15x15 Openbeam. Also doubles as an endstop/keeper for the MGN12 carriage.

See http://www.thingiverse.com/thing:431251 for the matching MGN12 carriage with integrated probe.

Timelapse video: https://www.youtube.com/watch?v=xO8zVhPIr4M