title: Django import-export fields
body: <p>I have a short question about django-import-export. In my model I have choice list:</p>
  
  <pre><code>STATE_CHOICES = ((NEW_STATE, u'New'),
                   (DELIVERED_STATE, u'Delivered'),          
                   (LOST_STATE, u'Lost'),
  </code></pre>
  
  <p>And method that handles mapping choices for names</p>
  
  <pre><code>@staticmethod
  def get_status_name_by_status(status):
      return next((s[1] for s in MyModel.STATE_CHOICES if s[0] == status), 'Uknown')
  </code></pre>
  
  <p>I want to import/export some data</p>
  
  <pre><code>class MyModelResource(resources.ModelResource):
      status = fields.Field(column_name='status', attribute='order',
                            widget=ForeignKeyWidget(Order, 'status'))
  </code></pre>
  
  <p>I want to use my get_status_name_by_status method so the choices will be converted to names. But there is no possibility to use method here, only fields are allowed. Any tip how this can be done ?</p>