This is so we can drop the dep on system python-urlgrabber, which will
block us from going to python3.
All we need is like 300 lines from python-urlgrabber for the progress
bar. In reality our needs are much lower, we don't need the fancy
progress bar that urlgrabber provides, but it's nice to have. So if
keeping a copy of this code causes issues in the future, we can probably
come up with something simpler (or hopefully there's a more common
python progressbar impl that we can use at that point).
Cole Robinson