Move Modernizr to the end of the page

Let's move the Modernizr script to the end of the page `</body>`, and inline the toggling of the `.js` and `.no-js` classes in the `<head>`.

**More info**
https://github.com/h5bp/html5-boilerplate/issues/1605
https://github.com/Modernizr/Modernizr/issues/878#issuecomment-41448059 