Switched to the stable/1.5 branch for next django version
Ported the code to run on Python 2 and 3.

This removes Django 1.3 compatibility. Django 1.3 users must stick to an
older version. Also removed some testing code that backported features from
Django 1.4 (assertTemplateUsed, assertHTMLEqual, HTMLParser).

Thanks @aaugustin and @claudep for the porting tips!