Remove Responsive Grid from Pure, defer to `rework-pure-grids`, add `.img-responsive` class

This pull request has the following things inside it:

* Removes grids-r.css, which removes all rules associated with `.pure-g-r`.
* Adds an `.img-responsive` class to `base.css`.

I've been using this branch locally to make the Pure-site mobile first, and use the new mobile-first grid system that's in the works.

## Removing `.pure-g-r`

This pull request removes `.pure-g-r` and it's associated media-query rules from Pure. This is due to the following:

* We want to minimize the number of media queries we have in Pure. Media Queries cannot be overwritten, so we think people should generate their own media queries.
* We have a better **mobile-first grid system** in the works, powered by [rework-pure-grids](https://github.com/ericf/rework-pure-grids). Since it's mobile first, we don't need `.pure-g-r` any more. 
* The new grid system will let you customize your media queries in a way similar to what was demonstrated [in this pull request](https://github.com/yui/pure/pull/146). This new grid system doesn't need to use `.pure-g-r`. 
* We are moving the media queries out of Pure, and making it easier for people to generate their own responsive grid , with their own media queries using the [rework-pure-grids](https://github.com/ericf/rework-pure-grids). We'll have documentation for this on the Pure site.

## Fluid Images

Since we are removing `.pure-g-r`, we need to have some way for folks to have their images be fluid. Before, this used to be defined in `grids-r.css` as this:

```css
.pure-g-r img {
    max-width: 100%;
    height: auto;
}
```

Since that no longer exists, I added an `.img-responsive` class, which has the same rules. Add this to your images if you want them to be fluid. 

## Things to do before merging this PR in

* [ ] Update grids doc. with details on new mobile-first grid system
* [ ] Update grids doc. with details on how to use rework-pure-grids
* [ ] Update Pure-site to use mobile-first grid and send PR for that
* [ ] Update layouts to use mobile-first grid and send PR for that