Django 1.7 compatibility

* Export signals handling to models.py
* Export all settings to one unique file
* Export keyfmt to utils.py
* We are not importing models anymore in __init__.py which can cause in several cases multiple fails from django.apps Registry
* Refactor unittests using django own DiscoverRunner

Currently all tests pass except SwitchTests.test_no_query which I'm missing something why we should make on query as we don't cache query from Django itself (we only cache proxies from ``waffle.__init__.py``).

I'm available if you want more information.

This PR is currently under development and not available to merge btw.