Simplify expanded menus

[slavanga]

Expanded menus are using quantity queries to define the item widths. This generates a lot of unnecessary CSS output. There is no need to define a width if table-layout: fixed is used.

Here’s a little demo: http://codepen.io/anon/pen/zvQXQq

[zurbrandon]

Yeah, this is something we're interested in. If you have time for a PR we'd LOVE that!