Take padding into account when positioning the tip

Fixes #4 and makes a note for #5.

Currently the code doesn't take into account the padding of the tip in its position calculations. The result is that the tip appears 10px off-center since the default styling adds `padding: 5px` to the tip.

I renamed `actualWidth/Height` to `tipWidth/Height` so you know that it's referring to the actual *tip* width/height.