Helper mixins + Groundwork tests

There's 2 parts to this pull request.

First is to update layout helpers so that intermediate web devs who have disabled $classes can now include helper mixins, rather than extend helper placeholders. The main reason for doing so is because extends fail to work in the following situation (due to a limitation with current CSS/SCSS):

.banner {
  @media only screen and (max-width: 767px) {
    @extend %pad-top;
  }
}

In this situation you want banner to be padded only on tablet devices or smaller. Instead,  banner is padded regardless of device width (i.e. placeholders don't work well with media queries).

However, the following would work:

.banner {
  @media only screen and (max-width: 767px) {
    @include _pad-top;
  }
}

Which is what this pull request enables. This should come in useful to people wanting to utilise Groundwork to customise a responsive layout. Note that helper placeholders are still around, so now web devs have 3 options for using a layout helper: includes, extends, or CSS classes (unless he's disabled them).


Secondly, the tests folder has been revised, mainly to check that recent changes to _grid and _helpers were working as expected without messing any old functionality up. So far so good; responsive grid placeholders and helper mixins appear to be working nicely. We can use this tests folder to test other components in future.

Note that to save space, compiled CSS files aren't included in the tests/ folder. You have to compile locally from tests/src/scss/(whatever).scss into tests/(whatever).css.