Django 1.7 support

This is heavily work in progress. 

The changes include: 
 - Add OSCAR_OVERRIDE_MODELS list to specify if a oscar model should be registered. In Django 1.7 we can't register models with the same name multiple times.
 - Don't register various dashboard.* modules as separate apps anymore. Duplicate app_labels aren't allowed. It might be a better idea to move the dashboard functionality for a specific app in the app itself (e.g. oscar.apps.catalogue.dashboard)
 - Add AppConfig objects as <app>.config. Django documentation uses <app>.apps but that might cause confusion with the <app>.app module.

Current test status:
 - unit: 4 errors, 1 failure
 - integration: 7 errors
 - functional: 169 errors