How to do an internal reference to the model itself?
Check out http://peewee.readthedocs.org/en/latest/peewee/models.html#self-referential-foreign-keys

tl;dr

``` python
added_by = ForeignKeyField('self', related_name='users_created')
```

I'm building a website in which Admins (which are of course also regular Users) can add new users. For this reason, my User model looks something like this:

```
class User(db.Model, BaseUser):
    username = CharField()
    password = CharField()
    added_by = ForeignKeyField(User, related_name='users_created')
```

Unfortunately, this throws a `NameError:  name 'User' is not defined.`

Is there a way to do this using peewee?
