Kossel magnetic endstop z-probe (zero x/y & low z offset)

Based on paul_delta's zero x/y offset probe (http://www.thingiverse.com/thing:1198256), with which I had the very first real success calibrating my Delta printer using this online calculator: http://escher3d.com/pages/wizards/wizarddelta.php by the user dc42.

Compared to paul_delta's design, this remix has some benefits:
- Lower z offset (paul_delta's design: ~ 120 mm, this design ~ 30 mm)
- Less mass -> less tendency to swing/vibrate due to high acceleration
- Low activation height (~ 0.5