Edit books without cloning a repository

This adds a second filesystem implementation that uses [octokit.js](https://github.com/philschatz/octokit.js) to edit and save books directly to GitHub.

Also, `node-webkit` supports [Native Promises](www.html5rocks.com/en/tutorials/es6/promises/); are there plans to remove Q?

Screenshot:
![image](https://cloud.githubusercontent.com/assets/253202/2874671/e7d613ca-d3f5-11e3-8958-9b703e044ccc.png)

Eventually, I would like to try and run the editor directly on `gh-pages` like [oerpub/github-bookeditor](https://github.com/oerpub/github-bookeditor) ([demo](http://oerpub.github.io/github-bookeditor)), a WYSIWYG eBook Editor.