Added OpenSCAD OSHW logo code

I'll put it on something, probably.
Starting work on the frame

I'll trim a lot of material out of the side blocks before I'm done.
The H-beams seem pretty decent, though.  I'll probably base the
trimmed-down sides on the H-beam code, actually.

I also added an assembled model with wooden dowels shown as
transparent gray cylinders.  It really shows how snazzy the plotter
is going to look.