Twenty Eleven: style.css cleanup and Showcase template improvements, props matveb - see #17198

 * Add full support for the featured slider powered by JS (except auto-slide)
 * Change the title hover state of large stickies to white
 * Prevent plain text posts from being displayed transparently over images
 * Update slider featured image size to 500x300
 * Add comment blocks to showcase.php

git-svn-id: https://core.svn.wordpress.org/trunk@17761 1a063a9b-81f0-0310-95a4-ce76da25c4cd