Mobile-first version of Pure's Website

**Note:** *This PR is a work-in-progress. There's still some stuff left to do (see list below). I've opened the PR to receive feedback while I finish up the remaining work. I'll remove this line once I feel that work is done so that we can do a final pass at that point.*

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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.
* The Grid documentation has been overhauled to talk about mobile-first responsive grids, fluid images and more. I also removed some of the older documentation that aren't needed anymore. See [990b51](https://github.com/tilomitra/pure-site/commit/990b51d0009229d4547af2029f3cba508788c777) for more details about this.

## What's left to be done

* [ ] 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.
* [ ] Once https://github.com/ericf/rework-pure-grids/issues/6 is fixed, we need to update `Gruntfile.js`.
* [ ] *Optional*: I think it would be nice to have a web UI for rework-pure-grids that is built into the Pure website, but this can be a v2. It would be nice to have routing set up to some useful pre-created grids. 
* [ ] Merge https://github.com/yui/pure/pull/267 into Pure when merging this in.