text: Title: Self referencing model
  Body: Hello and thx for your lib,
  
  I'd like to self-reference a model, but I'm not sure how to do it as I'm quite new to Python, what I do is:
  
  ```
  class Player(Document):
      created_on = DateTimeField(default=datetime.now, db_field='d')
      name = StringField(required=True, db_field='n')
  
  Player.friends = ListField(ReferenceField(Player))
  ```
  
  But I get an `AttributeError: 'NoneType' object has no attribute '_type'` in `python3.6/site-packages/graphene_mongo/converter.py`
  
  Am I doing it wrong or is it not possible?