Responsive grid placeholders

While building a site with groundwork yesterday I realized grid placeholders don't behave responsively like their counterpart grid classes. Have tried my hand at fixing it (along with reorganizing code in _classes and _grid).

There are still a few issues to be tweaked as noted in my latest commit, particularly the need for testing beyond the basic one I did up. But by and large it seems that grid placeholders now behave responsively (see /test/test.html). Would you like to pull this?