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!


Sorry no, `before` is the text before the selectionStart cursor in the text area. If I make a window selection (see screenshot below) then I would expect the `selection.before` property to contain "This is a " and the `after` property to contain " that may be selected".

![screen shot 2014-09-23 at 10 56 39](https://cloud.githubusercontent.com/assets/47144/4369654/90ee5918-42ff-11e4-8ebd-41e8abaa53e2.png)

This aside, after playing around with this feature creating the screenshot above I'm not convinced that this is good behaviour anyhow. The only real use case I can think for it is quoting a previous reply and I think this can be handled fine by copy and paste.

We could look at implementing something like this around the reply button. Where if you select some text before clicking reply it's pre-populated in the text area with quote markup.

Removing it also removes the `$window` dependency from the directive. So lets do that.