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

Finalize the implementation, fix scrolling for elements near the end of
the page, make sure the values calculated for top-offset and height are
accurate (and unaffected by box-sizing or offset-parents), add fallback
for IE8's lack of `getComputedStyle()`.
Also, remove two redundant calls to `$anchorScroll()` from the docs'
examples.
There still needs to be docs and tests.