Add box-sizing to columns

I think box-sizing: border-box should be added to columns to allow for borders and padding to be applied directly to .column/.columns without breaking the grid.

This should help cut down on the amount of markup required for the grid setup (in some cases).

Original commit message:
*Added box-sizing to columns to allow for borders, padding etc. to be
added inside the column, thus not breaking the grid if required.*

See here for current browser compatibility:
http://caniuse.com/#search=box-sizing