Added a mask layer to prevent mouse scrolling.

In certain cases when the datepicker is used on a section that has a scroll independent from the body, scrolling using the mouse wheel will result in the datepicker to remain still while the date input is moved, as shown in the following jsFiddle: http://jsfiddle.net/cesarFnord/yQf2r/

I added a mask layer that will prevent the mouse from scrolling to prevent this.