Fix .gitmodules, for real this time.

I had this done correctly but while preparing new branches for pull
requests, somehow this got mangled.
add .gitignore for generated files and *~
add setup scripts and Make rules for setup/generation

Previously the build procedure was documented via comments in
gendocset.py.  This CL automates much of the prep & build.  The
automated version now also uses virtualenv and pip, in line with modern
Python dev practices.
Fix sentence fragment in Makefile
Update packaged docset.tgz

Note that this was made with the package make rule.