I've been facing this problematic of 'how to build a quick prototype that allows you to do a much better and clever communication with your customer at meetings' rather than doing just mockups or flow simulated storyboards i've chosen to follow this stack:
Twitter Bootstrap (Just for management and inner control pages)
Backbone.js (Much easier to work with event handling for responsiveness and async non-blocking app sensation scenario)
EpicEditor (Keeping current state and modifications on real time edit is the simple way for feedback)
Annotator (recently added, keep track of your user comments is valuable)
Google App Engine SDK for Python
MongoDb at MongoLab (REST ready without extra config, and usable with urlfetch)
With this the performance on building a real functional prototype is huge, generating a first functional draft for 10 min per page, with a simple relaible draft design.