Hank's 3-point bed leveling for Printrbot

This printrbot print bed platform and base starts with Beardface's printrbot bed ( http://www.thingiverse.com/thing:17717 ) and incorporates a three point leveling system using the leveling knob idea from http://www.thingiverse.com/thing:9094 (reimplemented as an OpenSCAD file), but using a 3-point leveling approach. The bed platform and base are designed to be cut out of 1/4" MDF using a laser CNC printer. The spinner knobs can be 3D printed from ABS or PLA.

Caveats:
- Since the leveling