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!


@aron I didn't implement this _exactly_ as you described, but I did rename the functions and create an `insertMarkup(value, selectionStart, selectionEnd)` function that cleans up the code nicely. 

The `wrapSelecition(selection, markup)` was the one I was having problems with. I could get it to work fine for `applyInlineMarkup(markup, placeholder)` but couldn't write it in a way that made it reusable in other parts of the code, since the conditions for evaluating whether markup has already been applied are quite different for inline vs block elements.

Perhaps I missed something? Happy to take another stab if you want me to.