Use new urllib3 'stream' parameter.

In shazow/urllib3#186 I proposed to add a `.stream()` generator to the urllib3 response object. This avoids some of the nasty problems with the `.read()` method on that object (apparent zero-sized reads when there's still data, all sorts of stuff). It's also a much more natural way to stream data.

This PR would update urllib3 to include that PR, and updates our streaming methods to use it where possible.

@michaelhelmick: Do you want to try this branch with your twitter issue? Use one-byte chunk sizes.