Experimental support for Python 3.3

Some caveats:

As noted in #1632, some of our dependencies have not released a Python 3.3 compatible version yet, this includes:

* django-mptt (install from github master branch)
* django-hvad (install from github master branch)
* djangocms- plugins.

Also note that MySQL-Python currently does not support Python 3.

All tests pass (tested 2.7 and 3.3 on Django 1.5), not a whole lot of manual testing.