Refactoring to use React

Summary:
React (http://facebook.github.io/react/) is a new library that Facebook
released last week and it seems awesome. It's designed for defining
components declaratively and helps out with a lot of the stuff that we
were doing by hand before, like only updating the DOM when necessary.

I converted everything except interactive-graph (which I just haven't
gotten to yet).

This is 350 lines shorter (not counting React itself) and much much more
responsive (on my computer, rerendering when editing a hint takes around 2ms
per keypress instead of 60ms previously (!) because lots of DOM manipulation
and other recalculation is avoided).

Test Plan:
Used test.html. Added hints. Removed hints. Added widgets to the
question.  Modified widget properties (e.g., input-number width) and saw
the preview update. Added radio choices.

Used testrender.html. Called zk.scoreInput() in the JS console and got
results appropriate for the radio choices I selected. Called
zk.showHint() and hints appeared.

Reviewers: eater, alex

Reviewed By: alex

CC: joel, jakesandlund

Differential Revision: http://phabricator.khanacademy.org/D2510