Added Cross Origin Resource Sharing support.

In production, I host JS/CSS/IMG on a CDN (Amazon S3 in my case) Without this patch Pixi.js will not let me load images on another domain/server as textures. With this code, you can use the following example to load your assets from another Server/Domain as long as that Server is CORS-enabled.

```javascript
    loader = new PIXI.AssetLoader(asset_list);
    loader.onComplete = onAssetsLoaded;
    loader.crossorigin = true;
    // Other pre-load things you might need to do.
    loader.load();
```

This works for me, am I missing any other key places modification would be necessary?