Responsive Grids — Media Query Unit Generation

This generates allows people to generate custom unit sizes for responsive grids.

Create a customized set of responsive units for Pure Grids:

```js
var rework    = require('rework'),
    pureGrids = require('rework-pure-grids');

var css = rework('').use(pureGrids.units(12, {
    mediaQueries: {
        sm  : 'screen and (min-width: 30em)',
        med : 'screen and (min-width: 48em)',
        lrg : 'screen and (min-width: 64em)',
        xlrg: 'screen and (min-width: 75em)'
    }
})).toString();
```

Now use the new selectors in HTML:

```html
<div class="pure-g">
    <div id="main" class="pure-u-1 pure-u-med-3-4 pure-u-xlrg-2-3">
        <h1>Main Content</h1>
    </div>

    <div id="side" class="pure-u-1 pure-u-med-1-4 pure-u-xlrg-1-3">
        <h1>Side Content</h1>
    </div>
</div>
```

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### Todos

- [x] Make the API better. Only `units()` plugin with `queries` as options?
- [x] Make example in README.md better.