Python 3

This pull request enables python 3 by default in wagtaildemo. It currently depends on https://github.com/torchbox/wagtail/pull/173.

Changes include:
 - Switched to use the standard precise32 vagrant box (wagtail-base-v0.1 doesn't support python 3, I will fix this)
 - Removed unicode literals from migrations
 - Modified provisioning scripts to install python 3
 - Changed requirements to point to github repositories (as many of the requirements don't yet have a Python 3 compatible version on pypi)