Added offset_polygon() and offset_points() to utils.  Both depend on PyEuclid and are relatively naive approaches to offset point sets; they can't handle curves and will make stupid mistakes with non-convex shapes.
changed whitespace in utils.py for use in Textmate
Added explanatory docstrings for offset methods un utils.py
added invert argument to arc().  This is useful for fillets and rounds.
Added extrude_along_path().  May need some tweaking, but it's mostly there.