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 needs to remember the insert position and place the cursor back there, currently it jumps to the end of the text area. eg.

    offset = (text.before + markup + text.selection).length
    input[0].value = newtext
    input[0].setSelectionRange(offset, offset)