New: Support for `box-sizing: border-box;`

- Setting `box-sizing: border-box;` for table cells would cause
  DataTables to incorrectly calculate the size of the element when
  applying the scrolling draw. This is because jQuery's $().width()
  always returns the content width (taking into account box-sizing).

- One possible fix was to detect the box model used and switch between
  width() and outerWidth(), but a much better fix is to use
  $().css('width') as this does take into account the box-model and
  allows DataTables to draw the scrolling table columns correctly,
  regardless of the box model. It should actually also improve
  performance, since jQuery doesn't need to look the box model up
  itself.

- This fixes issue #157