title: Django: display nested categories in admin multiple select
body: <p>I would like to find a way to get categories and subcategories 
  displayed in the admin, in the form of a multiple select. </p>
  
  <p>Like:</p>
  
  <pre><code>parent 
  ----child1 
  ----child2 
  parent2 
  ----child3 
  </code></pre>
  
  <p>Do I have to make a custom field or is there already a solution 
  around? </p>
  
  <hr>
  
  <h2>Edit</h2>
  
  <p>the model is:</p>
  
  <pre><code>class Category(models.Model):
  
      def __unicode__(self):
          return self.name_en
  
      name = models.CharField(_('name'), max_length=255, null=True)
      slug = models.SlugField(_('slug'), db_index=True, unique=True)
      parent = models.ForeignKey('self', blank=True, null=True, related_name='child')   
      description =  models.TextField(_('description'), null=True)
  </code></pre>