hgroup is no longer valid

The hgroup tag has been removed from the html 5 draft specification[1].

For the meantime we'll use a div with class of "hgroup", may refactor in the future.

[1] http://lists.w3.org/Archives/Public/public-html-admin/2013Apr/0003.html
Merge pull request #165 from alphagov/remove-hgroup

hgroup is no longer valid