Remove hidden undo files.

These are created by your local editor and shouldn’t be committed to
the repo.
Exclude build/, dist/, and undo files.

This should make git automatically exclude these directories and
filetypes.
Add missing files to SOURCES.txt

setup.py added these, and they’re all actual files, so I assume they’re
useful.
Merge pull request #1 from durandal42/master

Cleanup / learning how pull requests work.