Python 3 support (take 2) [Part 1 of 2]

This pull request adds python 3.3+ support to Wagtail. Theres still a couple of test failures which I hope to solve soon but its mostly working.

To make reviewing easier, this  pull request doesn't contain any unicode literals changes so there is no support for python 3.2 or below yet. I will open a separate pull request for that.