Python 3 support

Django 1.5+ supports Python 3. The following commits allow me to run `python manage.py staticsitegen` using the Disk (or Base) Renderer without encountering any exceptions*, on both 2.7.3 and 3.3.0 using Django 1.6.

*the other renderers are semi-tested, insofar as they get imported, so I had to fix the import differences and syntax changes anyway.