Git Repo for Mendel

I've collected a full set of mendel parts, and replaced as many STLs as possible with existing OpenSCAD versions - including the latest versions of Vik's files and the OpenSCAD rework of Wade's geared extruder. I'm working to replace the remaining STLs with OpenSCAD re-works. Also on the todo list is unifying various configuration options into a single config file, and extracting more shared code into mendel_misc.inc.

I will gladly accept patches, pull requests, additional files, etc. Let'