Add offsettTop to offsetHeight for scrollHeader

If you have multiple fixed headers, it’s not enough to calculate the
headerHeight as the height of the sticky header element. We need to add
the offsetTop to compensate for the first sticky header.
Merge pull request #36 from follesoe/master

Add `offsettTop` to `offsetHeigh`t for `scrollHeader`. Allows for multiple fixed headers, or a fixed header that sits slightly below the top of the page.