progress Property, States & small fixes

Hi there,
I made the following changes:
• added a progress property (float between 0 and 1)
• fixed a typo (succes: -> success:)
• added a State property to determine basic info about the current progress (ready, downloading, failed, done)
• made the fileName possibly writeable (because when you are downloading files which come from a php script for example, the previous implementation always saved the file as "download.php". Now default is to use the last path component of the url. If the fileName is set manually, we use that)