[WIP] Django 1.7 Support

This pull request adds Django 1.7 support into Wagtail.

TODO:
 - [x] Rewrite migrations to use Django 1.7
 - [ ] Get unit tests succeeding (Currently blocked by https://code.djangoproject.com/ticket/22487)
 - [x] Tox setup
 - [ ] Test
 - [ ] Add pyinotify to wagtaildemo requirements (support for this was added in Django 1.7 and it makes reloading on file change faster)

Blockers:
 - [x] https://code.djangoproject.com/ticket/22487 - Preventing tests from loading initial data from migrations. https://github.com/django/django/pull/2643 fixes this (I've checked) but is not merged in to Django yet
 - [x] https://code.djangoproject.com/ticket/22680 - We have a workaround in place for the moment, but it would be best if this was fixed in Django
 - [x] https://code.djangoproject.com/ticket/22485 - Preventing us from running "makemigrations" on 1.7b2+
 - [x] django-compressor - Need version 1.4 which isn't released yet (hopefully this month)
 - [x] django-libsass - 0.1 doesn't support django-compressor 1.4