Screwbox - Bi

This is a screwbox based off of [Defauld's screw box](http://www.thingiverse.com/thing:582221), which I had some trouble getting to work well with my setup.

This version uses two broad screw threads and has clearance built in both parallel and perpendicular to the axis. Additionally, I've tried to make it less stabby by filleting and chamfering in the obvious places.