Add `em` support to max and min-width variables

In short what I would like is to set this in the variables

```
$min-width           = 25em
$max-width          = 50em
```

and get this in the css file

```
@media all and (min-width: 25em) {
  . . .
}
```

I think this could be a great addition, but I'm not sure if it's possible.

Here's the reason why I like to use them http://blog.cloudfour.com/the-ems-have-it-proportional-media-queries-ftw/
