Add some way to scroll an element

[mgreenland]

Hi,

I'm writing a test where I want to scroll an element (to test some infinite-scroll logic), but it doesn't look like there's any built-in way of doing that. The Mouse class doesn't seem to expose anything; there's a scroll method in the Touch class, but using that fails with "Underlying driver does not implement advanced user interactions yet.".

I can work around it by modifying the element's scrollTop manually, but it'd be nice to have something that'd trigger all the proper mouse/touch events.