Keep document on the same place when prepending

When scrolling up and prepending items, the currently visible
items are pushed down and you are losing what you are looking at.

We are now getting the difference in document height before and
after the prepending and we are adding it to the scrollTop property
of the window.
Resolve promise when finished in both directions

We are now resolving the promise only on destroy.
The promise would never be rejected.

The reason for this change is a undesired behavior when using
the infinite scroll in both directions.

Since only one promise is returned for both directions,
when items are all loaded in one of the directions and we resolve
the promise, progress notifications are no longer fired for the
other direction.