Passing an array with different image widths lets backstretch select the best suited -- and reload on resize accordingly

Usage:

```javascript
$.backstretch([
  [{width: 400, url: '/dog400.jpg'}, {width: 1200, url: '/dog1200.jpg'}],
  [{width: 500, url: '/cat400.jpg'}, {width: 1100, url: '/cat1200.jpg'}],
], {ease: 750, duration: 3000})
```

(But it also works without a cycle and as a single object with `width` and `url`, just like a url string would).