Cleanup

- Rename and move the variable to variables file
- Move code to the grid file
- Use the mixin to generate our own classes
- Wrap in a grid classes enabled conditional
- Document the mixin

scss/_variables.scss

@@ -215,6 +215,7 @@ $container-max-widths: (
 
 $grid-columns:                12 !default;
 $grid-gutter-width:           30px !default;
+$grid-row-columns:            6 !default;
 
 
 // Container padding