Give site a style refresh

What's there currently is hacked together by yours truly, not very well designed.

Here's what I'd like to get going:
1. Start with the `book` branch
1. The whole site needs a redesign
1. It should look good on mobile
1. The book contents itself is going online and needs better design
1. At the footer of each chapter file (see /book/chXX.html files) we should promote that the book is available for sale.

The plan is to put the entire contents of the book online in HTML form (while still promoting and selling the ebook formats). The theory is that making the book content open and google-able will actually improve sales and help more people adopt the tools and philosophy.

How it's all set up:
1. It's a static site served with github pages.
1. There are two very similar jade layout files in the `book` branch.
1. one for the book
2. one for the docs site: http://docs.humanjavascript.com/
1. The menu on the left is generated with JS by reading the `` tags on the page

How to build/run it:
`npm install & npm start` will run a static file server that serves the site on localhost:8000

Start from the book branch and inspect the two Jade layout files, the /book/chXX.html chapter files, and the JavaScript that builds the left menu from heading tags. Run npm install && npm start and review the site at localhost:8000, including mobile layouts. Done means the site and book pages have a cohesive responsive redesign and chapter footers promote the book's sale.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.