ANCHOR SCROLL: WIP - feat($anchorScroll): add support for configurable scroll offset

Add support for a configurable scroll offset to $anchorScrollProvider.
The offset is expressed in pixels and can be specified either as a fixed value or as a function that returns the offset it pixels dynamically.
(This is a POC and a WIP: no docs, no tests)

Related to #9368

Closes #2070, #9360
I believe that we can use `getBoundingClientRect()` for both `top` and `height` properties. See http://jsfiddle.net/98gr0w7p/ and http://jsfiddle.net/98gr0w7p/1/

According to MDN it works pretty much everywhere for our purposes https://developer.mozilla.org/en-US/docs/Web/API/Element.getBoundingClientRect

This would remove quite a lot of computation both here and further down