Slow loading if no images found.

When the page has no images, the plugin wait for the `maxTime` to complete, instead of just completing right away.

Example:
http://jsbin.com/dureti/edit?html,output - Contains 1 image, loads pretty fast.
http://jsbin.com/henaba/edit?html,output - Has no images, takes really long time to complete.

What if we add a check after `getImageSrcs` function on https://github.com/Gaya/queryloader2/blob/183e0c0ae129300982c8e68e5eaba1f0c87c0d41/src/ImagePreloader/index.js#L29

``` js
if (this.sources.length === 0) {
this.done === true;
this.updateProgress();
}
```