Add last_modified and last_modified_id to StorableObject

* qommon/storage.py:
   those methods can be used to help user-agent caching of page
   depending solely upon the content of a type of objects, like much of
   the admin pages.

wcs/qommon/storage.py

@@ -330,3 +330,13 @@ class StorableObject(object):
     def remove_self(self):
         self.remove_object(self.id)
 
+    def last_modified_id(cls, id):
+        filename = os.path.join(cls.get_objects_dir(), fix_key(id))
+        stat = os.stat(filename)
+        return stat.st_mtime
+    last_modified_id = classmethod(last_modified_id)
+
+    def last_modified(cls):
+        stat = os.stat(cls.get_objects_dir)
+        return stat.st_mtime
+    last_modified = classmethod(last_modified)