cas: write download progress to stdout

This introduces ProgressReader, a io.Reader proxy that keeps track of
the bytes read and fires off a callback with the current state.

Fixes #208


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There are lots of different ways of doing this. I don't know if there is any precedent for writing to stdout yet, but this seems okay for now.
"iop" makes me think of iops, how about just sticking with ioprogress?