Update stylesheets

* Remove unused CSS rules.
* Make consistency changes.
Remove the `html5shiv.js` and the need to use it

The `index.html` file only uses 3 of the new HTML5 semantic elements.
These elements don't really bring much benefit, and because older
browsers require extra code in order to display them correctly, they
also add to the overall page weight.

This commit replaces these elements with `div`s and removes the
`html5shiv.js` file (that has a filesize of 9504 bytes or 1245 bytes
minimized and gzipped), reducing the overall page weight and elimineting
1 extra component that needs to be maintained.
Add `Disallow:` to `robots.txt`

Ref: https://github.com/h5bp/html5-boilerplate/commit/4e5f438e1ea0d7e39179ba5cf799a8d326ba7cc3.
Add `/browserconfig.xml` and tile images

Ref: h5bp/html5-boilerplate#1481.