title: ViewFlow and django-guardian
body: <p>I want to make use of <code>django-guardian</code>'s  object permissions and grant specific rights for specific users to one or more Django users.</p>
  
  <p>I have tried to add some permissions to my Process class like this:</p>
  
  <pre><code>class TestProcess(Process):
      title = models.CharField(max_length=64)
  
      something = models.ForeignKey(ForInheritage, null=True, on_delete=models.CASCADE)
      no_approval = models.BooleanField(default=False)
      approved = models.BooleanField(default=False)
  
      def something_is_approved(self):
          try:
              return self.something.approved
          except:
              return None
  
      class Meta:
          permissions = (
              ('view_process', 'View Process'),
          )
  </code></pre>
  
  <p>Unfortunately this causes <code>viewflow</code> to immediately throw an error after starting <code>runserver</code>: </p>
  
  <pre><code>File "/home/me/.virtualenvs/viewflow3/lib/python3.4/site-packages/viewflow/mixins.py", line 253, in ready
      self.flow_class.process_class._meta.permissions.append(
  AttributeError: 'tuple' object has no attribute 'append'
  </code></pre>
  
  <p>My initial plan was to subclass <code>Start</code> and <code>View</code> flow classes to change how the <code>Permission</code> function, that is inherited from the <code>PermissionMixin</code>, works. But this seems to be more work than just this, too.</p>
  
  <p><code>django-guardian</code> is already mentioned in one of the <code>cookbook</code> sections <a href="https://github.com/viewflow/cookbook" rel="nofollow noreferrer" title="here">here</a> but currently leads to a 404 page. </p>
  
  <p>What would be the recommended/cleanest way to use guardian permissions on Processes and Tasks?</p>