Improve urlopen

[dopplershift]

Currently, this is what urlopen does:

BytesIO(create_http_session().get(url, **kwargs).content)

As we start to use this more, it would be good to provide an implementation that's smarter and only downloads bytes on request (when possible), using things like HTTP Byte-Range requests.

[dopplershift]

For reference:

• httpio
• s3io
• Stdlib io module has BufferedIOBase