Make index.html pass the W3C HTML5 validator

- hgroup has been removed from the HTML5 spec:
  http://lists.w3.org/Archives/Public/public-html-admin/2013Apr/0003.html
- An img element must have an alt attribute
- "&" has to be escaped as &amp; (this includes "&" in URLs!)
- Escape Unicode character
Merge pull request #56 from DasBlub/gh-pages

Make index.html pass the W3C HTML5 validator