Rearrange modules in pure.css rollup to: Base, Grids, etc.

This moves Grids to be the second module (after Base) in the pure.css
rollup files.
Improve CSSLint config to work on a per-module basis

The grunt-contrib-csslint Grunt plugin does odd things with the way it
processes CSSLint config. This improves linting by only applying in-file
lint overrides on a per-module basis.
Add Base to src/ which is Pure's extras on top of Normalize.css
Add `[hidden]` selector to Base to counteract all `display` declarations

Anytime a Pure style rule uses the `display` declaration with a value
other than `none`, using the `hidden` HTML attribute on that element
won't have the desired affect of hiding that element. To counteract
this, A new rule has been added to Base to force
`display: none !important;` on elements that use the `hidden` HTML
attribute.

The following button will be hidden from view with these changes:

    <button class="pure-button" hidden>No showy</button>

Fixes #177
Merge pull request #256 from ericf/hidden

Add `[hidden]` selector to Base to counteract all `display` declarations
Update README.md

Add iOS 7 to list of supported browsers.
Bump rework-pure-grids dependency

Fixes #260