Fixes for two Expression bugs.

(sin^2(x)) is now properly parsed.
x/(x+y) now renders without parens.

Test Plan:
src/expressiontools/test.html

Auditors: alpert
Categorizer variable height.

Auditors: alpert
Fix large category placeholders.

Auditors: alpert

Sorry for the double audit, I discovered this problem later.
Focus widgets in question

Auditors: jakesandlund
Background image for graphs.

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D3222