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 actually knew a lot of this (and was going to ask about it), but for some reason `MathJax.Hub.Que` doesn't work when called in `ctrl.$render`, unless you call it with `scope.$apply`, which also throws an error (even though the math renders). I was going to ask you about this as I knew that doing it the way I did it was bad.

I just had a realization though: I was trying to call MathJax under `scope.$watch 'preview'`, and MathJax rendered perfectly fine on Preview but the rendered html was lost as soon as it was saved. 

I also noticed, calling MathJax on `scope.$watch 'editing'` doesn't work when you press save the first time, but if you select edit again and then save, the math renders. Does that mean there is something weird going on in `annotation.html`? I see 4 uses of the `<markdown>` element. So scope.$apply must be working because the Markdown being displayed in the preview box vs. the actually annotations fall in different scopes (since apply is all scopes)?