Use set_password in case of the instance is AUTH_USER_MODEL

I want to use model_mommy to create user_models too like an example as follows.

```python
user = Recipe('myapp.User', username=seq('User Name'), password=('password'))
```

(However, I wonder if we should use `django.contrib.auth.get_user_model()` or set_password when the instance has `password` attribute.)