title: How to return __str__(self) in ModelAdmin list_display
body: <p>I have the following model:</p>
  
  <pre><code>class Administrator(models.Model):
      // code
  
      def __str__(self):
          title = 'Administrator ' + str(self.pk)
          return title
  </code></pre>
  
  <p>This returns titles for the objects as <code>Administrator 1</code>, <code>Administrator 2</code>, etc.</p>
  
  <p>How do I return the <code>title</code> in <code>list_display</code> for a <code>ModelAdmin</code>?:</p>
  
  <pre><code>class AdministratorAdmin(ModelAdmin):
      model = Administrator
      list_display = ('pk', 'id', 'title???')
  </code></pre>