hgroup element is obsolete

The `hgroup` element has been removed from HTML5 and should not be used. References:
* http://www.sitepoint.com/html5-hgroup-element-dropped/
* http://www.w3.org/html/wg/drafts/html/master/obsolete.html#non-conforming-features
* http://www.w3schools.com/tags/tag_hgroup.asp

Therefore I replaced the obsolete `hgroup` elements with `div` elements and adjusted the CSS.