Re-styled <MathOutput>s, which now convert innermost dfracs to fracs

Summary:
<MathOutput>s are now more compact and better-spaced. They also convert innermost dfracs
to fracs, using similar logic to the parseTex transform we implemented for <InputNumber> and
<NumericInput>. All of that logic is now bundled into a single file, which I've named TexWrangler
(let's bikeshed on that).

Test Plan:
- Run Perseus.
- Create this [item](http://localhost:9000/test.html#content=%7B%22question%22%3A%7B%22content%22%3A%22%5B%5B%E2%98%83%20input-number%201%5D%5D%22%2C%22images%22%3A%7B%22https%3A%2F%2Fka-perseus-graphie.s3.amazonaws.com%2Fda8df81c78b22f5c69d477d8eabfb583968eaf84.png%22%3A%7B%22width%22%3A400%2C%22height%22%3A70%7D%2C%22https%3A%2F%2Fka-perseus-graphie.s3.amazonaws.com%2Fb59fc02ca1aae800977b8793ed22f647a1aa75ee.png%22%3A%7B%22width%22%3A425%2C%22height%22%3A150%7D%7D%2C%22widgets%22%3A%7B%22input-number%201%22%3A%7B%22type%22%3A%22input-number%22%2C%22graded%22%3Atrue%2C%22options%22%3A%7B%22value%22%3A2.5%2C%22simplify%22%3A%22required%22%2C%22size%22%3A%22normal%22%2C%22inexact%22%3Afalse%2C%22maxError%22%3A0.1%2C%22answerType%22%3A%22number%22%7D%2C%22version%22%3A%7B%22major%22%3A0%2C%22minor%22%3A0%7D%7D%7D%7D%2C%22answerArea%22%3A%7B%22type%22%3A%22multiple%22%2C%22options%22%3A%7B%22content%22%3A%22%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%7D%7D%2C%22calculator%22%3Afalse%7D%2C%22itemDataVersion%22%3A%7B%22major%22%3A0%2C%22minor%22%3A1%7D%2C%22hints%22%3A%5B%5D%7D) with correct answer 2.5. Run `itemRenderer.setInputValue(["input-number 1"], "2 \\dfrac{1}{2}", false);` in the console.
    - Verify that it grades correctly (i.e., that the parseTex transform still works).
    - Verify that the fraction is small.
    - You could also try a nested fraction, but input-number doesn't support grading those anyway, so you'd have to log the output to verify that it's correct.
- Run `itemRenderer.setInputValue(["input-number 1"], "2 \\dfrac{\\dfrac{1}{2}}{3}", false);` in the console.
    - Verify that the outermost fraction is large (dfrac), but the numerator is smaller (frac).
- Run the mobile app with this build of Perseus and verify that inputs look appropriate
    - "Properties of multiplication" is a good exercise with many consecutive inputs that used to overlap.

Reviewers: jack

Reviewed By: jack

Subscribers: alpert, emily

Differential Revision: http://phabricator.khanacademy.org/D11910