Auto-generate man pages

This adds two commands: `python setup.py build_man` and `python setup.py install_man` using the `help2man` program. I did not make them run by default, since I guess it's not usually used, and it requires an external program.

I have opened this as a PR, since I hope someone else can do something about the Debian packages. I can add the `help2man` dependency, but I don't know how to get it to call the additional commands.