Make it possible to assign customer model to other things besides User

Initial backend support, partially taken from django-registration. The
default is already working, need to do something about admin.py,
missing some tests. It includes functions for the management commands, as well as create customer, get customer and create customer from user.
I don't quite understand why the related model is hardcoded to be User. Should this not look like this:

```python
DJSTRIPE_CUSTOMER_RELATED_MODEL = safe_settings.DJSTRIPE_RELATED_MODEL
```
..with a corresponding section in safe_settings.py?