Bugfix & new function to fix a special case

7555a89: (Bugfix) $window.last was assigned to container before. This caused the condition in the if statement on line 31 to always be false.

3c7fb22: Added useDocumentBottom option to fix issue with absolute positioned children. When elem has absolute positioned children, the height of elem is 0. The causes the scroll function to run for every scroll event, even when it's not even near the bottom of the page (it went bananas). Absolute positioned children is very common in masonry grids, so this is very useful.

**Note**: I haven't bumped the version or included the build. I though I'd let you do that, so it's done the way you want it :)
**Note regarding note**: I didn't know that commits made after a PR has been opened were included afterwards, oops.