Replace truncate_words by Truncator.chars

This fixes a lot of deprecation warnings in Django 1.5. Truncator is
supported in both Django 1.4 and Django 1.5.

The original code could result in double ellipsis. This commit simplifies
the logic and only relies on Truncator.chars which feels like a better
way of truncating the text anyways.
Remove code which was needed to support Django 1.3
Add myself to the AUTHORS file. Yay! :-)