Twenty Twelve template changes and fixes ... see #19978.

 * Add custom header support
 * header.php: use `get_stylesheet_uri()` to load stylesheet, props Mamaduka
 * Add full-width template, including adding relevant body_class value as a CSS hook
 * Hide sidebar on all views if no active widgets exist
 * Fix site title output (was missing site title)
 * Remove "featured" title for sticky posts
 * Add content template files for aside, image, link, and quote post formats
 * Indent content-single.php to match content.php
 * Remove sample JPG since Twenty Twelve ships with no default header images

... and style updates, props drewstrojny

 * Better footer styles
 * Styling for full-width layouts
 * Add basic styles for archive views
 * Refine post header/footer styles a bit


git-svn-id: https://core.svn.wordpress.org/trunk@19915 1a063a9b-81f0-0310-95a4-ce76da25c4cd