The value of top option is not considered when determining the visibility of
the element. The element is scrolled after it's top offset is lower than the
$window.scrollTop() position. Setting the top value to a bigger value e.g.
200px shows a "jumping" effect: when scrolling the element "jumps" to it's
correct position 200px below the browser window after it scrolled off the
window.
This commit fixes the behaviour. The top value is considered as a margin-top
of the element, so it stays visible the whole time.