Twenty Thirteen: pinking shears

git-svn-id: http://core.svn.wordpress.org/trunk@23780 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Thirteen: style up the new `mediaelement` audio and video players. Props obenland, fixes #23821.

git-svn-id: http://core.svn.wordpress.org/trunk@23781 1a063a9b-81f0-0310-95a4-ce76da25c4cd