Progress bar is ignored and not provided if url contains "dropbox.com"

[poke1024]

Not only does this look like a hack (in terms of some non-dropbox urls containing "dropbox.com" somewhere in their name), it's also quite unfortunate if you download large files from dropbox.

https://github.com/choldgraf/download/blob/99c51c50b935f8125d20fbc13399d29f3eaf8e35/download/download.py#L188-L204