Python 3 support

Adds Python 3 support (3.3 specifically). I'm mainly copying/paraphrasing @jacobian's [rationale here](https://github.com/jamesturk/django-markupfield/pull/13)..

- No support for Python 3.0, 3.1, or 3.2 -- it's not worth the added effort. Anyone using Py3 is on the cutting edge anyway, so getting them to use 3.3 is fine.

- Drops support for Django 1.3 because Django 1.3 doesn't have `django.utils.six` available. I did not specifically remove the [1.3 stubs](https://github.com/idlesign/django-sitetree/blob/master/sitetree/admin.py#L15). If 1.3 support is needed let me know and I will work on that.

- Drops support for Python 2.5. I'm not sure 2.5 and 3.3 support is possible in a single source tree, but again if it's important I'll give it a shot.