Twenty Eleven: Style fixes and showcase improvements; Props matveb

* Remove bottom border from last post
* Improve featured posts on the showcase: move slider navigation to the top right; make sure the bottom border shows; make the reading more link thicker
* Positioning fix for image post format meta
* Dark stylesheet improvements
* Ensure search input text is centred vertically


git-svn-id: https://core.svn.wordpress.org/trunk@18050 1a063a9b-81f0-0310-95a4-ce76da25c4cd