screw_thread.py: fixed bug in internal neck-in
-- Fixed off-by-one bug in screw_thread.py, removed comments.
-- Updated unit test.
-- Fixed bug in transform_to_point() that caused bad transforms when dest_normal and src_up were parallel.  This also fixed a problem in extrude_along_path()
-- Updated test cases
-- Added split_body_planar() and split_body_[horizontal | vertical]()
-- Unit tests for split methods
-- Added path_extrude_example.py
-- Updated README with path_extrude, offset, and screw info
-- Fixed a bug in scaling in extrude_along_path()