Move add_to_class functions to their own module

* The new module is called methods.py and found on each app.
* Add keyword arguments to add_to_class instances.
* Remove catch all exception handling for the check in and
  check out views.
* Improve checkouts tests code reducing redundant code.

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

mayan/apps/acls/classes.py

@@ -24,7 +24,9 @@ class ModelPermission(object):
             app_label='acls', model_name='AccessControlList'
         )
 
-        model.add_to_class('acls', GenericRelation(AccessControlList))
+        model.add_to_class(
+            name='acls', value=GenericRelation(AccessControlList)
+        )
 
     @classmethod
     def get_classes(cls, as_content_type=False):