Column Header widths troubles

Fixes:
* widths like 30px/30em are internally stored as 30p/30e
* css width on header ignored (attribute still works)
* header calculation wrong when css box-sizinig: border-box and scrolling enabled

Set up that's broken:
http://live.datatables.net/kivekin/2/
Same Set up including my fix:
http://live.datatables.net/kivekin/3/