add git commit date to version string via gitattribute filter...

To fix https://github.com/jedie/django-tools/issues/1 i have change the way how the git commit date would be added to the version string in django-tools with: https://github.com/jedie/django-tools/commit/c72661de2e358370bf5e7a7ad575f12e0ec7f75c

I created a simple git filter in python for this. The filter any many information can be found here:
https://github.com/jedie/python-code-snippets/tree/master/CodeSnippets/git/