Added ability to add XHR progress listeners to HttpReader.

Hi,

Firstly, thanks for creating this library.

I was playing about a bit trying to download large zips, and I thought it would be a good idea to be able to get progress updates from the HttpReader.  So I added this.

For the test provided I was downloading a local copy of "dojo-release-1.8.3-src.zip", which is a 38Mb zip.  It took 3 or 4 seconds to download, even from localhost - hence wanting to see progress updates.

Maybe you'd like to pull this in, or create something equivalent.

Thanks.