Update html.

* switch to HTML5
* add meta charset
* add html lang
* move js before the end body tag
Move inline style to csso.css.
Reindent HTML for better readability.
Use https for GitHub's links.
Add images to the repository and compress them losslessly.

Refactor selectors.
Refactor files in a more standard way.
Add normalize.css for better cross-browser support.

The CSS is minimized with the csso web interface.
Fix validation error.
Move inline JS to javascript.js.
Switch to a `ul` for the contributors list.
Remove inherited color.
Make use of the HTML `footer` and `small` tags.

Also remove an unneeded `br` tag.
css/csso.css: refactor selectors.
Switch to class for `container` and break long line.