20140321 - Scrolling background

Added code to jquery.backstretch.js which scrolls the height of the
background image from page top to page bottom.

Also: some changes to .ui-show and .ui-hide in luv.css.
Split the definitions up further to cover the differences in the way
transitions are used across the layout.
20140323 - Scrolling coolbackground

Nicholas F from France, via Fiverr, rewrote Backstretch.js.