Twenty Eleven: showcase template improvements - see #17198 (props kawauso for patches)

 * Only show "featured-posts" area if there are published sticky posts.
 * If there is only one sticky, hide the slider and just display the single featured post.

git-svn-id: https://core.svn.wordpress.org/trunk@17846 1a063a9b-81f0-0310-95a4-ce76da25c4cd