Add new $grid variable for max number of columns

Set limit on max number of columns to be included in style sheet using $grid variable.

Rewrote responsive grid mixin to set limit on unnecessary left, right, skip classes and set as a joined style to prevent code bloat. This gives the same output as before without the additional classes, while avoiding the endless amounts of identical styles created using a standard loop.

Grid variable does not currently affect new grid adapters.