WIP: Add unit test infrastructure

Travis is now configured (through `.travis.yml` and `config-tests.yml`) to be able to:

- generate the HTML of a book (unit test)
- apply the CSS to the HTML
- compare the HTML (for regression-testing changes)

**Note:** The "baked" `style="..."` attributes are sorted alphabetically (using [philschatz/css-diff.js](https://github.com/philschatz/css-diff.js))