Refs #626: Drop Django 1.3 testing, add Django 1.5 testing

Updated tox.ini to not test with Django 1.3 anymore, and to
start testing on 1.5.  Got that working. Tried to run tests
on trunk but that's not working yet. For example, the
'django.contrib.markup' template tag library is gone in 1.6
and we use it.

Branch: django15
Merge pull request #626 from caktus/django15

Drop Django 1.3 support