Add method to grant and revoke access via ACLs.

Granting will also check if the permission has been
authorized to the object class using ModelPermission.

Signed-off-by: Roberto Rosario <roberto.rosario.gonzalez@gmail.com>

mayan/apps/acls/classes.py

@@ -26,6 +26,10 @@ class ModelPermission(object):
 
         model.add_to_class('acls', GenericRelation(AccessControlList))
 
+    @classmethod
+    def get_for_class(cls, klass):
+        return cls._registry.get(klass, ())
+
     @classmethod
     def get_for_instance(cls, instance):
         StoredPermission = apps.get_model(