Sass 3.3 compatibility updates

- Fixed extends from within media queries.

-Added wide-button placeholder.

-Removed a number of grid__item extends, greatly reducing compiled code
output.

-Added tab documentation.

-Fixed classed headings bug where classes were getting nested.

- Made type-headings mix in more responsive.

- Media-query mix in now accepts multiple breakpoints.

- Gitignoring sass cache files.