Twenty Thirteen: add support and styling for Audio structured post format. Moves audio content out from the main post content, using `the_audio()` and `the_extra_content()`. Props obenland, fixes #23871.

git-svn-id: http://core.svn.wordpress.org/trunk@23828 1a063a9b-81f0-0310-95a4-ce76da25c4cd