Twenty Eleven: style improvements, RTL, and more header images - see #17198

* Add two more default header images (see r17729)
* Change the featured posts limit from 4 to ten in front page slider
* First pass at robust RTL styles
* Pingback style fixes
* Wider comment form for the one column layout
* Add CSS comment with instructions on how to remove the post format headings


git-svn-id: https://core.svn.wordpress.org/trunk@17790 1a063a9b-81f0-0310-95a4-ce76da25c4cd