add support for hidden=true in randomize xmodule

Support a new 'hidden' xml attribute for <problem>. If set to 'true' the
problem will be excluded from the list of choices within the randomize
xmodule.

common/lib/xmodule/xmodule/randomize_module.py

@@ -102,6 +102,8 @@ class RandomizeModule(RandomizeFields, XModule):
         if self.choice is None and no_repeats:
             history = json.loads(self.history or '[]')
             children = [c for c in children if c.location.url() not in history]
+        children = [c for c in children if not
+                    self._str_to_bool(c.xml_attributes.get('hidden', ''))]
         return OrderedDict([(c.location.url(), c) for c in children])
 
     def get_choice_index(self, choice=None, choices=None):