Introduce zero-dependency Markdown Editor

Dokku: http://markdown.dokku.hypothes.is/
This is an alternative approach to https://github.com/hypothesis/h/pull/1478

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)