Updates to the _s_admin_header_image() callback

Adding a ".displaying-header-text" class to the header h1 and to the site description div inside _s_admin_header_image(). This allows the "display text with your image" toggle button to work in the admin preview under Appearance > Header.
Minor code style clean-up
Add trailing commas to the last elements of arrays
Ditch license.txt from theme and change License URI to canonical source of information on version 2 of the GNU General Public License. Also adjust theme license DocBlock to explicitly state that _s is GPL v2 or later.
`nav-` prefix for css class 'previous/next'.

Re-introduce nav- prefix for previous/next links (see 6056c1e).
nav- prefix for css class 'previous/next'.
Fixes the broken display header text option in the Customizer.
Merge pull request #202 from gatespace/works

Add missing `nav-` prefix for 'previous/next' links in CSS.
Remove hgroup from theme. It's dead now: http://www.sitepoint.com/html5-hgroup-element-dropped/. In its place a div will be use that inherits the old class that was being applied to header.
Merge pull request #201 from iamtakashi/minor-clean-up

Minor code style clean-up.
Restore site-header class to header element and give site-branding class to title and description wrapper div. See #206.
Turn post thumbnail support off by default, as it's used nowhere in the theme by default. Also add a DocBlock into the theme which gives more information on post thumbnail support implementation.
Merge pull request #206 from Automattic/rip-hgroup

Remove hgroup from theme. It's dead now.
_s: Removes unnecessary switch in and out of PHP.
Merge branch 'master' of https://github.com/Automattic/_s.git
Remove  title attributes where it duplicates  in anchor text. See http://core.trac.wordpress.org/ticket/24203.
Merge pull request #209 from Automattic/remove-redundant-title-attributes

Remove `the_title_attribute` title attributes where it duplicates `the_title` in anchor text. See http://core.trac.wordpress.org/ticket/24203.
DocBlock adjustments to post thumbnail support. See #207.
Merge pull request #207 from Automattic/post-thumbnails-off-by-default

Turn featured thumbnail support off by default.
Fix unlinked site title in the Customizer
Remove HTML5Shiv from _s. See #215.