Shell might be finished?

I believe shell is finished, but I also realized it has some problems
with thin parts. I'll improve it after I get the whole thing working a
little bit, since it's easily treated as a black box.

Most of this is bugfixing:
- added mesh.__str__
-Found an error in edge.setDir that isn't fixed yet "IndexError: invalid
index to scalar variable"
-added some .all()s that were missing in Basics
-Accounted for the shape of the array representing v in
plane.vector_intersect
-Created main.py, just a way to pull everything together for testing.
-Refactored stl_importer.py from the first attempt. It now works on
current code.

When I try to use mesh.chop I'm somehow getting some floats tossed into
the output lists. I need to track down what's doing that. I've
eliminated stl_importer as the cause.