Twenty Thirteen: add back-compat function to avoid activation with older WordPress installs. Props kovshenin,  see #23819 and #13780.

git-svn-id: http://core.svn.wordpress.org/trunk@23816 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Thirteen: better styles for Chat structured post format output. Props Joen for starter CSS, closes #23869.

git-svn-id: http://core.svn.wordpress.org/trunk@23817 1a063a9b-81f0-0310-95a4-ce76da25c4cd