Django 1.7 support

All the bugs blocking the previous attempt to add Django 1.7 support seem to be fixed now.

Wagtail has moved on so much since then, so this pull request starts adding Django 1.7 support again from scratch.

- [x] Create initial schema migrations
- [x] Create initial data migrations
- [x] Update ElasticSearchQuerySet to use Django 1.7 where nodes
- [x] Get all tests working (one more test to go)
- [ ] Test

How you can help:
 - Test Wagtail with Django 1.7 features and let us know if you have any problems
    - use ``pip install -e git://github.com/kaedroho/wagtail.git@56e4dac#egg=wagtail``
    - or add ``-e git://github.com/kaedroho/wagtail.git@56e4dac#egg=wagtai`` to your requirements.txt
 - *FIXED* ~~Theres a couple of places where Wagtail loops over INSTALLED_APPS. This doesn't take into account INSTALLED_APPS can now contain Django 1.7s AppConfigs (https://docs.djangoproject.com/en/dev/ref/applications/#for-application-users). These need to be changed to take these into account.~~
  -  ~~https://github.com/torchbox/wagtail/blob/100797796df0bc8ca96035092f32a9275d2b3713/wagtail/wagtailcore/hooks.py#L43-L49~~
  - ~~https://github.com/torchbox/wagtail/blob/100797796df0bc8ca96035092f32a9275d2b3713/wagtail/wagtailimages/formats.py#L88-L94~~
 - *FIXED* ~~There's a test that is failing on Python 3.4. It looks like a unicode/bytes issue: https://travis-ci.org/torchbox/wagtail/jobs/31053446#L308~~