add setIndex method. see #9.
possibly fix max autoPadding height+border problem

An element with a border has a child with no height or width properties, but a
left and top of 0: this causes the child elements background to cover the
bottom and right borders. It doesn't have a `position.right`, but `iright`
still needs to be subtracted.
do not scroll if detached. fixes #8.
minor fix. potential fixes.