Performance improvements

* Only set #background[style.postion='absolute'] once.
* Reduce DOM access/calculation for scrolling.
* Disable background opacity transition during scroll.
* Use el.style instead of el.setAttribute('style') for translateY
* Use +new Date instead of new Date().getTime()
* Resize background manually, as width: 100% was causing uneccessary repaints.
* Use margin/padding: 0 instead of width/height: 100% for html CSS rule.

Would also benefit from removing the Twitter iframe, but have left that up to you :) Hope it helps!