Add refresh() method

Here's a quick'n'dirty `refresh() ` method for refreshing the list of images.

Our site has an AJAX load more thingie and–naturally–I wanted to utilize this little script in the load more'd content also. Therefore, I needed a way to refresh the array of images after adding the new content to the page.

Example usage:

```javascript
var adaptiveBacks = AdaptiveBackgrounds();

$.ajax(url, args).then(function (data) {
  $container.append(data.html);
  adaptiveBacks.refresh();
  adaptiveBacks.process();
});
```

:hamster: