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. 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.