Generated grid system with responsive support

Grid system is now with generated classes and with responsive support #16 

This means that no longer can we &:extend grid columns in the implementation, instead we can use .makeGridColumn(4) for ex.

Later we might add another mixin to generate all classes manually to support extend but for now let's keep it like this.

Note: This is a breaking change so be careful about updating.
Probably you should update the comment. The new minWidth is 414px