RFC: Update generate-at-breakpoints mixin

Allow a mix of min and max media queries. eg 
```
@include generate-at-breakpoints('.u-position-fixed', (400 max, 401 min, desk max)) {
  position: fixed;
}
```

Change max-width media queries generated by the mixin from this format `.u-position-fixed-at-400` to `.u-position-fixed-up-to-400`


See my comments in Scally HipChat room about this:

![screen shot 2014-10-08 at 8 27 24 am](https://cloud.githubusercontent.com/assets/1342772/4550671/c6055222-4e68-11e4-8a26-aa787dc35bec.png)

@kllevin what do you think about this?

I think even if we don't change `min-width` mq's we definitely need to update the general mq `max-width` mixin so we're being consistent.