django 1.7 : tests, compatibility

So many broken things :cry:, but now fixed :smile: :

**NOT READY** : there is still some work on migrations left.

```
# in tests
AppRegistryNotReady: Apps aren't loaded yet

# in my projects, problems with admin forms:
The value of 'form' must inherit from 'BaseModelForm'
```

Changes:
 * deprecations : get_query_set, model.module_name
 * app loading : must use some late imports and introspection
    * we can no longer use ``model._meta.get_field_by_name``, which uses a cache that is not yet initialized at setup, but we can use ``model._meta.get_field`` instead. **There might be a performance difference.**
 * settings : default middlewares changed, so we no longer rely on them
 * metaclasses : copy updated method from six for django 1.5 to 1.7 compatibility
    * six.with_metaclass only supported 1 metaclass and 1 base class; but django 1.7 requires the forms to inherit from BaseModelForm, which requires to have multiple base classes, hence the copied method to provide compatibility across all supported django versions
 * Tests for other versions still pass.


**There was absolutely no effort to check db migrations, so this might be a problem**