Twenty Thirteen: improvements to Image post format handling to use a custom image size instead of filtering a `$content_width` variable.

Includes CSS cleanup and markup improvements to both video and image templates, moving the media HTML piece out of the `header` element.

Props obenland, closes #23620.

git-svn-id: http://core.svn.wordpress.org/trunk@23839 1a063a9b-81f0-0310-95a4-ce76da25c4cd