Add math support

This PR adds math support. It uses a new and very light math rendering library from Khan Academy called [KaTex](http://khan.github.io/KaTeX/).

However, since the library is new and [lacking support for some Tex commands](https://news.ycombinator.com/item?id=8320439), I've also implemented a MathJax fallback which will load MathJax from a CDN if KaTex throws an error in rendering. Khan Academy is also doing something similar apparently. 

Dokku = https://math-support.dokku.hypothes.is




This only checks for **one** math part in the text.
So if I have something like that:
```
$$c = \\pm\\sqrt{a^2 + b^2+d}$$

Wonderful

$$c = \\pm\\sqrt{a^2 + b^2-d}$$

Both are true.
```

It'll be rendered like this:
![math_card](https://cloud.githubusercontent.com/assets/2094134/4418822/ad2f5aee-4563-11e4-8f29-b893c0de4262.png)