Add Box sizing variable to grid

To not break existing layouts when changing their `box-sizing` to `border-box` a variable of `$grid-box-sizing` was added to the _grid.sass.

Setting `$grid-box-sizing` to `border-box` will tell crispy that the project is using the `border-box` box-model. Crispy will then skip the recalculation of the column width if there are any old padding and border-width properties set in the mixin.

When starting fresh projects you should simply set your box-model to `border-box` and simply don't worry about setting any border or padding-widths to your column mixins.