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/xmodule/progress.py

@@ -13,6 +13,8 @@ class Progress(object):
 
     Progress can only represent Progress for modules where that makes sense.  Other
     modules (e.g. html) should return None from get_progress().
+
+    TODO: add tag for module type?  Would allow for smarter merging.
     '''
 
     def __init__(self, a, b):