Show that create_content_type modifies current context

I am trying to register a content type on two models:
```python
ModelOne.register_regions(('main', 'Main'))
ModelOne.create_content_type(MyContent)
ModelTwo.register_regions(('main', 'Main'))
ModelTwo.create_content_type(MyContent)
```
But I am receiving an error from the final line:
```
ImproperlyConfigured: Cannot create content type from non-abstract model (yet).
```
This is because `MyContent` is getting redefined in my `models.py` on the second line.

I must admit I am finding this one very hard to debug, but I suspect it is caused by one of the calls to `type()` in `feincms.models`.

The attached diff has a failing test for the problem.

Do you know what may be causing this? I would love to help if I can, but I am a little stumped as to what action to take.