title: Zinnia Blog doesn't use custom auth user model
body: <p>I am using Zinnia Blog module with my Django app. I have a Django app and have a custom User model. I have added the following setting:</p>
  
  <pre><code>AUTH_USER_MODEL = 'account.Member'
  </code></pre>
  
  <p>However, when I try to create a new entry in the blog I get the Foreign Key constraint error. </p>
  
  <pre><code>Exception Type: IntegrityError
  Exception Value:    
  (1452, 'Cannot add or update a child row: a foreign key constraint fails (`example`.`zinnia_entry_authors`, CONSTRAINT `author_id_refs_id_410656e2874b02ff` FOREIGN KEY (`author_id`) REFERENCES `auth_user` (`id`))')
  </code></pre>
  
  <p>It references the default auth model (which doesn't exist in the database) instead of the specified auth model. How do make Zinnia use the correct table.</p>
  
  <p>Thanks.</p>