title: Making a CharField use a PasswordInput in the admin
body: <p>I have a Django site in which the site admin inputs their Twitter Username/Password in order to use the Twitter API.  The Model is set up like this:</p>
  
  <pre><code>class TwitterUser(models.Model):
  screen_name = models.CharField(max_length=100)
  password = models.CharField(max_length=255)
  def __unicode__(self):
      return self.screen_name
  </code></pre>
  
  <p>I need the Admin site to display the <code>password</code> field as a password input, but can't seem to figure out how to do it.   I have tried using a <code>ModelAdmin</code> class, a <code>ModelAdmin</code> with a <code>ModelForm</code>, but can't seem to figure out how to make django display that form as a password input...</p>