add script for converting git log to GNU ChangeLog

found via:
http://git.661346.n2.nabble.com/Generating-GNU-style-Changelog-from-git-
commits-td6279979.html
which led to:
https://raw.github.com/cryos/avogadro/master/scripts/gitlog2changelog.py

Change-Id: I8693405a2ce9009618c0081e2084014e22cb54e5