Find an element's offset, then add it to the offset of the parent
until we are at the direct child of parentEl.
Use-case: find scroll offset of any element within a scroll container
Fixes#521. Reordering now uses webkitTransform instead of
element.style.left. Additionally, as you drag the drag-element to the
top or bottom of the scroll-area, it will scroll it up or down as
allowed.
Refactors necessary: Common code from `<content>` and `<scroll>` moved
into js/ext/angular/controllers/ionicScrollController. Then `<content>`
and `<scroll>` expose the controller, and `<list>` can require it.
`<list>` then uses the controller (if exists) to pass the scrollView and
scrollEl to ReorderDrag, and ReorderDrag uses that to scroll.
Additionally, js/ext/angular/test/controller/ionicScrollController tests
much functionality that was untested before.
