Tex rendering fixes.

Summary:
Fixed Mul.tex() rendering per Ivan's bug report.
-9^2 was rendering as "- \cdot 9^{2}", now as "-9^{2}"

Also refactored the hint system to be chainable.
We now better distinguish between negative signs and negative ones.
Added more tests.
Changed all fractions to use \frac with a prepended \displaystyle.

Test Plan: src/expressiontools/test.html

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D2649