Add MathJax CDN

This PR adds MathJax, so we can render math written in LaTex or MathML:
![hypothesis](https://cloud.githubusercontent.com/assets/521978/4208235/51973c86-385e-11e4-8f9f-747d1d0d8426.jpg)

Give it a try on: https://mathjax.dokku.hypothes.is/ 
Note: the markup for math consists of two dollar signs: `$$LaTex or MathML here$$`

This PR is an alternative to including the very large MathJax library in our vendor folder (as implemented in https://github.com/hypothesis/h/pull/1480). 
I don't think you need the `scope.$apply` here. There's a `$digest` on the immediately previous line. That means there are two digest cycles here now. The second one is also a full digest of all scopes (apply is all scopes, digest is this one and children only), which almost certainly isn't necessary. Also, IIRC MathJax replaces the HTML so what you really want is to do this only when the control is re-rendered. It's not necessary to do it when the input changes, because we're showing the source rather than the rendered output. So inside `ctrl.$render` I think you want to call `MathJax.Hub.Queue` unless `scope.readonly`.

Finally, just calling it as you've called it will cause MathJax to scrape the entire page again looking for Math. You want to provide `elem[0]` as the third element of the Array passed to `MathJax.Hub.Queue` so that MathJax only re-renders this element.