Fixed: simplified the overlay scroller test, documented the technique.

Previously the inner element's offsetWidth was tested before and after setting the overflow to scroll. This is unnecessary, we can just set the overflow to scroll and test the outer element's clientWidth vs. offsetWidth. By definition, clientWidth does not include scrollbars, whereas offsetWidth does. So if they are equal then overlay scrollers must be in use.

Note that FireFox (as of version 22) does not support overlay scrollers, so even if the system does, Cappuccino will not on FireFox unless the programmer forces overlay scrollers.