Basic support for items of different heights

Update example.
There can be an unpleasant flickering on desktop devices when items are
different heights.
Remove unneed code. Improve IE compatibility.
refactor
refactor ghost to work like placeholder

Insert it when it is used, and remove it when it is no longer needed.
Don't create it at the beginning then hide it when it shouldn't be
shown.
Add rackup info to readme
remove dependency on jQuery browser