IE8 with cross-domain XHR requests gives "Line: 184 Error: 'loaded' is null or not an object"

This is a fix for a problem with the "progress" event in IE8 when using XHR and CORS requests.

In my fork I've added a copy of `PreloadQueue.html` called `PreloadCORS.html`, and a `.htaccess` file to enable CORS headers on an Apache httpd server, as a way to reproduce the problem on IE8. To reproduce:

1. Reset your local branch to commit `1ba26b4ec417090cca5bc3cec0954b561890837e` (i.e. to revert my fix)
2. Configure httpd to point to the PreloadJS root directory
3. Add `AllowOverride All` to enable the `.htaccess` file
4. Configure `ServerAlias preloadjs.crossdomain.dev` for your virtual host
5. Add preloadjs.crossdomain.dev to your system's hosts file
6. Load `http://<your-domain>/examples/PreloadCORS.html` in IE8 (note: `<your-domain>` should NOT be `preloadjs.crossdomain.dev`).
7. IE8 should report `Line: 184 Error: 'loaded' is null or not an object`.
8. Pull in the last commit on my fork, and the error should no longer occur in IE8.