Added additional class for half size columns on mobile view

Usage: **.grid_6_mobile**

Currently only set up to work alone or in pairs. Alone will float a 48% width column to the left, in pairs will include a second column floated to the right.

Overrides default 100% width column on mobile view.

```html
<section class="row">
    <article class="grid_2 grid_6_mobile">2 Desktop - Half Mobile</article>
    <article class="grid_2 grid_6_mobile">2 Desktop - Half Mobile</article>
</section>
```