Fix .stretch for elements with padding and border-sizing: content-box

I'm getting larger-then-normal slide heights when I use `.stretch` with elements that have vertical padding and `box-sizing` set to `content-box`. The vertical padding isn't being taken into account and the height of the stretched element ends up being too large.  This pull request looks at the setting of `box-sizing` to decide whether or not to include border & padding in the calculation.