Add support for mobile grids as in Foundation

It would nice to be able to set column sizes for elements separately in the case of mobile layouts (481px+). As in Foundation where we use. 

class="small-2 large-4 columns"
To set the column size to 2 in the mobile version and 4 in the desktop version.

I've added small-onecol, etc. I know that this may not be the best implementation but I'm sure you can make a better version and bake it into bones.

I've added the mobile grid only in the LESS folder.