Fix #2 - Mobile browsers are unable to detect whether the entire page is scrollable

Adds useWindowAsViewport config option to allow the document to stretch
the containing element to its full size and use the window as its
viewport instead of using overflow:scroll on the viewport element.
This allows mobile browsers to hide browser chrome and jump back to
the top natively.
Merge pull request #8 from box/use-window-as-viewport

Fix #2 - Mobile browsers are unable to detect if the entire page is scrollable