Introduce zero-dependency Markdown Editor

Dokku: http://markdown.dokku.hypothes.is/
This is an alternative approach to https://github.com/hypothesis/h/pull/1478 
_(Let's try to keep discussion over there.)_

It introduces a simple Markdown editor to help users with no knowledge of Markdown easily insert links, make lists, etc.
![hypothesis](https://cloud.githubusercontent.com/assets/521978/4194840/3bbabd48-37b4-11e4-8cab-cf2cae7b25ad.jpg)

### Features
+ Extremely light weight, there are no extra dependencies added. 
+ Buttons for bold, italics, link, image, quote, code, lists, numbered lists, and LaTex or MathML.
+ Keyboard shortcuts for bold, italics, and link.
+ The buttons are pretty smart. For example, if you select some text and press bold, it will insert the markup around the text you selected. Try them out!


This could probably be renamed to something like `userSelection`. What I don't understand here from reading the code is why if the user has selected text that the `before`, `after` etc values still try to extract values from the text area, surely this is then wrong? You'll end up with `before + selection + after` being `{textarea text}{window selection}{textarea text}`. I'd be inclined to just remove the `$window.getSelection()` code for simplicity here.