Math render

TODO:

- [ ] Depends on #1 being merged Use [exercise-task...math-render](https://github.com/openstax/tutor-js/compare/exercise-task...math-render) to see the diff
- [ ] Decide on HTML format for encoding the Math
- [ ] Add tests


# Screenshot

![image](https://cloud.githubusercontent.com/assets/253202/4888223/92596db8-638b-11e4-98fd-5b1430e385ed.png)

# Format

Currently, Math is encoded as `<span data-math='\\dfrac{-b \\pm \\sqrt{b^2 - 4ac}}{2a}'></span>`. This has a few advantages:

- the unformatted Math can be rendered in gray text (`[data-math]:not(.loaded):before { content: attr(data-math); }`)
- a Loading spinner can be added into the span (CSS is similar to previous option)