windowScroll - fix for IE
windowScroll - fix for IE

In IE11 [`window.scrollY`](https://github.com/obogo/ux-angularjs-datagrid/blob/master/src/addons/windowScroll.js#L11) is not available and `undefined`. Thus will the datagrid compile from top to bottom instead of to compile the visible items on scroll.
[MDN](https://developer.mozilla.org/de/docs/Web/API/Window/scrollY) suggests to use `window.pageYOffset` for cross-browser compatibility.
It is reproducible with the windowScroll [sample](https://rawgit.com/obogo/ux-angularjs-datagrid/master/samples/window/index.html).
obogo/ux-angularjs-datagrid,obogo/ux-angularjs-datagrid