Add support for multiple grid sizes

[kjbrum]

Make it so you can pass a set of grid sizes to use, as well as set the default grid size:

// Create classes for 3 different grid sizes
$grid-sizes: (3, 12, 16);
$grid-size-default: 12;

Examples:

.gi-x-6 {
   width: 50%;
}
.gi-x-2-3 {
   width: 66.66666%;
}
.gi-x-3-12 {
   width: 25%;
}
.gi-x-10-16 {
   width: 62.5%;
}