Add `minkowski` and `color` transformations and general clean up

I added the `minkowski` and `color` transformations (hopefully correctly -- I basically copied `union` and `rotate`, respectively) and I removed some OpenCV stuff that was (seemingly?) left over from earlier text-related work. I also reorganized `scad.clj` and `model.clj` to more closely mimic the OpenSCAD documentation (calling things Boolean operations, transformations, etc. and changing the relative positions of definitions). Oh, and I removed the designs folder, as it references paths that don't generally exist (`/home/mfarrel/...`) and might be confusing. It'd be great to have a more general set of examples for people new to this project, though.

If this is too much to change all at once, let me know and I'll break it out into smaller pieces and submit them separately.