Kossel Mini carriage for MGN12

Added a probe for an optical endstop, re-enforced the backstop for the captured toothed belt and countersunk the four mounting bolt heads.

**WARNING**: See Instructions.

See http://www.thingiverse.com/thing:450238 for the matching optical endstop mount for Openbeam 15x15.

Timelapse video: https://www.youtube.com/watch?v=VxSCTOD9BZQ