title: Django ModelMultipleChoiceField fields to display from its queryset
body: <p>I have a ModelForm with a ModelMultipleChoiceField.    </p>
  
  <pre><code>users = forms.ModelMultipleChoiceField(
    widget=forms.SelectMultiple(attrs={'class': 'chzn-select'}),
     queryset = User.objects.filter(...))
  </code></pre>
  
  <p>My list is displayed by username, as the default <code>repr</code> method of the model.</p>
  
  <p>I want to display the <code>first_name</code> and the <code>last_name</code> without overloading the <code>repr</code> method, because I need username in other cases.</p>
  
  <p>I wouldn't like to get the <code>values_list('id', 'first_name', 'last_name')</code>, because later I don't get a model list on <code>save()</code> but tuples.</p>
  
  <p>Is there any setting/hack for this ?</p>