Ability to pass a window object

I'm using Steady across documents where I have JS in one iframe and the HTML in a sibling iframe. `window.requestAnimationFrame` doesn't seem to call the passed function in Firefox (31). I added an optional `opts.window` parameter (defaults to `window`) which is used instead of just assuming `window`.

If you can think of a way to get the `window` from `opts.scrollElement` then lets do that and we won't need a new parameter. I tried but it seems there's no easy way of getting the `window` from `scrollElement.ownerDocument`.