Consolidate grid generation code

- 2 almost-identical sets of grid generation code exist, one for classes in _classes.scss, and one for placeholders in _grid.scss. Consolidated them in _grid.scss. The new code will always generate for placeholders, and generate for classes if $classes = true.
- Similarly, shifted container/row/column/centered class generation from _classes.scss to _grid.scss. grid-classes() mixin is now obsolete.
- Thus, removed grid-classes() include in groundwork.scss.
Merge branch 'test' into responsive-grid-placeholders
Minor: Recompile test.css

Recompiled test.css after merging separately-developed test and responsive-grid-placeholders branches.

The only effect the code reorganisation has had on test.css is to change the ordering in 1 of the compiled lines, which is inconsequential, and is good. Am not going to look into why this swap happened.
Make grid placeholders behave responsively.

- grid-responsive() mixin now generates both placeholders (always) and classes (optional).
- Shift grid-responsive() mixin from _classes.scss to _grid.scss.
- Edit _responsive.scss to always include grid-responsive().
- Recompiled test.css and groundwork.css based on these changes.

Unresolved issues:
- _grid line 92 & 97: Haven't found a way to mimic :not(.mobile) for something that's not given the %mobile placeholder. On that note, how would mixing of classes & placeholders (e.g. .one.third%mobile) work??
- _grid line 90, 119, 131, 143: Seems like it would work, however, ugly as hell. Can't find a better way to do it as of yet.
- Haven't actually tested .right-n, %right-n, and their left and skip counterparts.
- _grid line 50 onwards: @for + nth() is probably refactorable using @each.