Twenty Fourteen: implement an alternate "slider" view for home page featured content. Props iamtakashi for the design and implementation. Slider JavaScript code adapted from FlexSlider v2.2.0 props WooThemes and mbmufffin. See #25550.

git-svn-id: https://develop.svn.wordpress.org/trunk@25979 602fd350-edb4-49c9-b593-d223f7449a82