Prusa i3 Printed Frame

A fully printed frame for the Prusa i3, inspired by http://www.thingiverse.com/thing:146061.

Key changes:
* Frame sections fully closed
* Different style of section interlocking for alignment, using M8 bolts running through frame for strength.
* Only drill holes for Z axis mounts included, as the other drill holes are mainly for zip ties and make limited sense with such a thick frame (an alternative is to use sticky back cable clips).
* Different electronic mounting holes (moved up