title: Custom permissions on viewset
body: <p>Im trying to find a way to create custom permissions on a viewset.
  Im want to implement somthing like this :</p>
  
  <pre><code>class ActivityViewSet(viewsets.ModelViewSet):
      queryset = Activity.objects.all()
      serializer_class = ActivitySerializer
      if request.method == 'PUT':
          permission_classes = (permissions.IsOwner)
      elif request.method == 'LIST':
          permission_classes = (permissions.IsAdmin)
      else :
          permission_classes = (permissions.AllowAny)
  </code></pre>
  
  <p>IE : sorting permissions by method. (the above code doesnt work cause "request" isnt recognized)</p>
  
  <p>Couldn't find any use in the "Custom permissions" section of the documentation, but maybe Im just blind. (overriding BasePermission? how? who? where?)</p>
  
  <p>Thanks :)</p>