Support for Django 1.6 django.contrib.comments deprecation

Adds support for projects using django_comments to Zinnia. django.contrib.comments was deprecated as of Django 1.6.

This patch should make Zinnia work with both the old django.contrib.comments and the new django_comments package.

To reproduce the error without the patch, just update the demo project using the following instructions and then run syncdb or some tests. You should get the exact error mentioned in #267. Which this patch addresses.
http://django-contrib-comments.readthedocs.org/en/latest/porting.html