Upgrade to python 3
Make this a module (and installable)
Refactor single file into discrete files

* author.txt was renamed to AUTHOR
* use git shortlog instead of git-shortlog
  because the latter is not necessarily in PATH

Signed-off-by: Heikki Hokkanen <hoxu@users.sf.net>

This should make gitstats work on FreeBSD.

Thanks to Alexander Botero-Lowry.