title: Django admin list filter
body: <p>I want to add a custom model method to the admin filter, however it fails.</p>
  
  <p>Example Foo:</p>
  
  <pre><code>class Foo(models.Model):
       number = models.IntegerField()
       def big_enough(self):
          return self.number &gt; 99
  </code></pre>
  
  <p>now at the admin panel:</p>
  
  <pre><code>class FooAdmin(admin.ModelAdmin):
       list_filter = ('number', 'big_enough')
  </code></pre>
  
  <p>Fails, I get the error </p>
  
  <p>ImproperlyConfigured at /admin/test/foo/
  'FooAdmin.list_filter[0]' refers to field 'big_enough' that is missing from model 'Foo'.</p>