Low Fastener Mendel

This is not completely hashed out, and is slightly smaller than a standard Mendel. But it should allow a frame to be constructed from these vertex modules. I've made special vertices for the top of the frame for the X motor and idler as the carriage http://www.thingiverse.com/thing:9385 can now slide along the top bars.

The Y axis components (still being worked on but in the SCAD file) slide on the lower two bars, so mechanical requirements are simple. The sockets are parametric and will work