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