[checkoutdownload] New version

After working with a rrhode, we've come up with a new version of checkoutdownload that allows different checkout products to specify different sizes for downloads (and hence it's easy to set different prices for different sized images.

Also while preparing the file for download the screen now shows a progress bar.

There is quite a lot of refactored code in this module so I'd be grateful if a few people could try it out, stretch it a little, see if it breaks.

Many thanks

ps Although I've labelled it 1.3 beta, I've not changed the version number internally so you should be good to go back to the old version without any grief.