Twenty Thirteen: hide gallery captions from small screens and when a gallery has 7 columns or more. Props kwight, fixes #23769.

git-svn-id: http://core.svn.wordpress.org/trunk@23700 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Thirteen: better caption styles for short words and sentences. Props kwight, fixes #23773.

git-svn-id: http://core.svn.wordpress.org/trunk@23701 1a063a9b-81f0-0310-95a4-ce76da25c4cd