Don't rerender answer area examples every time

We were calling MathJax a lot. Reusing the same DOM element avoids this.

Auditors: alex, emily
Make a draggy card orderer

Summary:
Add a widget that allows users to pull cards from a bank of cards and
order them to make an answer.

Test Plan:
Make an exercise, add the ordering widget. Add some cards to the
correct answer, and some other cards to the other cards. Make sure that the
numbers, symbols, and variables are sorted into different sections. Drag some
cards into the box. Make sure that they go in the right places. Make sure that
they animate going to the right places. Remove some cards from the box, try
putting them back into the box, and above back into the bank. Make sure they
disappear when they go into the bank. Make the correct answer out of the cards.
Make sure it is scored as correct.

Reviewers: jakesandlund, alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D2821
Upgrade React to fix IE8 bugs

Using 83ff6b6 from https://github.com/facebook/react/pull/139.

Auditors: emily
subjsx
Perseus multiple choice items now wrapped in <label>s for easy clickeroo.

Summary:
Perseus multiple choice items now clickable via their text as well as their radio/checkbox controls.

SIGNIFICANT NOTE: this is my first foray into React-land and I am 150% positive that there are much nicer ways of doing this. Consider this my plea for mentorship!

Less significant note: I was able to test this via test.html (the test plan below) but my grunt setup is giving me trouble, so this has not been tested after minification/uglification/etc.

Test Plan:
Run python -m SimpleHTTPServer 8083 in Perseus directory
Hit http://localhost:8083/test.html
Create answer type "Multiple Choice" and fill it in w/ some example answers
Verify that, on the right-hand preview, clicking the text of the example multiple choice answers selects the controls.
Verify that this does not happen when clicking/editing on the left side.

Reviewers: alpert, emily

Differential Revision: http://phabricator.khanacademy.org/D2863
Indentation cleanup, thx @alpert
"make all" should do a build and put only, no get.
Start of category table: dragging off bank.
Basic moving in and out of table, no styles.
Remove placeholder.
Renderer in category-table, and some refactoring.
Now able to edit some things.
Category editor fully operational.
Check answers on category table.
Make a double wide column for the odd column. Fixed sizes.
Add pretty styles and change name to categorization.
Snap to top, same width cards.
Bank on left, tables on right.
Add a categorization widget.

Test Plan: Will update this after animations are added.

Reviewers: alpert, emily

Differential Revision: http://phabricator.khanacademy.org/D2856
Make a real bank that acts like the table columns.