Sturdier printed Z axis for prusa i3

This is a printed frame for the prusa i3

The design seeks to improve upon the rigidity of earlier designs by [kknapp](http://www.thingiverse.com/thing:118020) while still taking less time to print than designs by [chocjulio66 et al.](http://www.thingiverse.com/thing:146061). The frame can be assembled incrementally - only the corner braces are absolutely necessary, the rest can be printed out and tacked on to provide additional rigidity.

The frame is built around a skeleton of six M8 threa