Fixes scollTop property that is deprecated on body

Chrome 32 deprecates document.body.scrollTop, should use
document.documentElement.scrollTop instead. Polyfill describe here
https://developer.mozilla.org/en-US/docs/Web/API/Window.scrollY
Style and reduce redundant code
Merge pull request #110 from jwestbrook/cumulativeScrollOffset-fix

Fix use of `scrollTop` property in `Element.cumulativeScrollOffset`.