Update the Twenty Eleven footer. Remove 'Theme:', translate 'powered by' and...

Update the Twenty Eleven footer. Remove 'Theme:', translate 'powered by' and add a title per Twenty Ten, add twentyeleven_credits action. props zeo for initial patch. fixes #17493.

git-svn-id: https://core.svn.wordpress.org/trunk@17954 1a063a9b-81f0-0310-95a4-ce76da25c4cd