title: How dynamic add custom field to model
body: <p>How add custom field dynamicly?
  I'm trying that, but the field won't insert into database when I sync db:</p>
  
  <pre><code>#It use as register(MyModel)
  def register(model, attr="my_attr"):
  
      if model in registry:
          raise AlreadyRegistered(
              _('The model %s has already been registered.') % model.__name__)
  
      registry.append(model)
      setattr(model, attr, MyField())
  
      MyField().contribute_to_class(model, attr) 
  
  #MyField.contribute_to_class
  def contribute_to_class(self, cls, name):
       super(MyField, self).contribute_to_class(cls, name)
  
       setattr(cls, self.name, self)
       cls.add_to_class('%s_manager' %  name, MyDescriptor())
  
       signals.post_save.connect(self._save, cls, True)
  </code></pre>