Import gitlog2changelog.py (2008-12-27)

https://github.com/cryos/avogadro/commit/8be9957e5b3b5675701ef2ed002aa9e718d4146e#scripts/gitlog2changelog.py
Issue #4: generate ChangeLog from git logs

This seems to generate long ChangeLog entries in the format we had with svn2cl,
but some commits appear to be missing.