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.
Build a test for testing responsive grid placeholders

This test quickly tests whether 1) changes to code are screwing things up and 2) grid placeholders are responsive yet.

Use the test by viewing test.html and resizing your browser.
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.
Merge branch 'master' into responsive-grid-placeholders
Merge pull request #59 from Sneakertack/responsive-grid-placeholders

Responsive grid placeholders