Mobile-first version of Pure's Website

This pull request is overhauls the site's CSS to be mobile-first, in accordance with the new mobile-first responsive grid system that we are building out. This pull request is closely tied to the work that I've been doing in the Pure repo in [PR #267](https://github.com/yui/pure/pull/267). That's the version of Pure that I've tested this stuff against.

## What's Changed

* `main.css` is now mobile-first, using `min-width` instead of `max-width`. The biggest change here is that the slide-out menu is now coded in a mobile-first way.
* Created a `Gruntfile.js` which generates the media queries for a mobile-first grid by using [rework-pure-grids](https://github.com/ericf/rework-pure-grids). 
* I'm in the process of removing all `pure-g-r` from the site. This is not needed anymore in the mobile-first grid. All instances of `pure-g-r` have been replaced with their mobile-first counterpart. Still need to remove `pure-g-r` from layouts though.

## What's left to be done

* [x] Add old-IE support
* [x] Clean up layouts to be mobile-first. They could each use their own grid, or maybe they can inherit from the site-wide grid. Haven't decided yet.
* [x] Once https://github.com/ericf/rework-pure-grids/issues/6 is fixed, we need to update `Gruntfile.js`.