Twenty Thirteen: enqueue Genericons CSS file for use in visual editor styles. Props DrewAPicture, closes #24595.

git-svn-id: https://core.svn.wordpress.org/trunk@24541 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Thirteen: minor CSS cleanup for navigation, props gorgoglionemeister. Closes #24651.

git-svn-id: https://core.svn.wordpress.org/trunk@24542 1a063a9b-81f0-0310-95a4-ce76da25c4cd