Update Download to address #632

Thanks @ccoupe for putting this together

Based on my understanding of the conversation in #632 this now does everything we want download to do in Shoes4.

I updated the manual accordingly. I'm thinking of replacing the deleted example with one that shows how to use the progress proc, but I'm on the fence since simple-downloader.rb is a pretty good tutorial in itself.

My only concern is that for some reason it is still possible for the simple-downloader progress bar to quit before it gets to 100.

Steps to repeat:
  1) Run simple-downloader.rb
  2) Try to download this example file http://ipv4.download.thinkbroadband.com/5MB.zip
  3) The download goes fine, except the progress bar doesn't quite reach the end.