Add support for LaTex and MathML via MathJax

This PR adds MathJax, so we can render math written in LaTex or MathML. 
![hypothesis](https://cloud.githubusercontent.com/assets/521978/4195326/66c48808-37bd-11e4-85e6-eba174f9b197.jpg)

Give it a try on: https://mathjax.dokku.hypothes.is/ 
Note: the markup for math consists of two dollar signs: `$$LaTex or MathML here$$`

The library is very large due to the size of the fonts. However, MathJax.js is very smart about loading those resources and will only load them if there is math on the page. More information can be found at [MathJax.org](http://www.mathjax.org/).