Initial progress display.

* add module_from_xml param to I4xSystem
* use it to implement xmodule.get_children()
* fix a few comments here and there
* Render-time progress display for seq and vertical modules.
  - Computes fraction of subproblems done.
* Pass problem state back to js during ajax calls.
* general cleanup in capa_module.py
* add progress_changed and progress fields to json returned from each ajax handler

* Coffeescript changes to hook up sequence tracking of problem progress

* net result: sequence 'a' tags now have a progress class
* properly set css class on initial load
* fire event when progress changes after ajax calls
* also save state in 'progress' property of problems-wrapper tag
* event handler finds those tags, computes updated progress

common/lib/capa/capa_problem.py

@@ -169,7 +169,8 @@ class LoncapaProblem(object):
     def get_score(self):
         '''
         Compute score for this problem.  The score is the number of points awarded.
-        Returns an integer, from 0 to get_max_score().
+        Returns a dictionary {'score': integer, from 0 to get_max_score(),
+                              'total': get_max_score()}.
         '''
         correct = 0
         for key in self.correct_map: