Fix issue with mouse position when page is scrolled

This deals with an issue where the mouse position is off by the scroll amount when the document is scrolled.

I simply switched the scroll from being added to the client to being subtracted from the page, since we're now always using the client offset.

See https://github.com/Leaflet/Leaflet/pull/1826 
and 
http://jsbin.com/etonoh/2 vs http://jsbin.com/etonoh/1