Fixes bug when scrolling and displaying an overlay for a position:fixed element

This is quickly put together pull request, just to start.

As noted in the issue #156:

moved the code that creates an overlay at a given position, to the callback after the scrolling animation has ended
removed html from $("html, body"), so that we are animating only one element and the callback is called only once (what was the reason for animating both html and body? )
no test for the animation behavior yet (apart from a visual test - last step in docs site)
At the moment there is still a problem with the #license step, if we arrive from a different page, because .offset() for this element differs between the document load and end of scrolling (this is most likely due to some other content on the page changing height/being inserted above #license). One solution for that would be to do the offset check again at the end of animation.

Tell me what do you think. Sorry if I missed something, don't have much time on my hands currently.