Retries take 3

This updates and expands on your patch a little bit. It looks like your branch is a little out of date with master... There's a better diff here: https://github.com/kevinburke/requests/compare/retries-take-3

This adds a ResponseError exception type for errors that occur while parsing the response from the server (also [described here][errors]). It's currently only raised if you pass in a custom Retry object - you can see an example usage [in the tests][tests].

A more aggressive change would make the ResponseError inherit directly from IOError instead of from RequestException.

[errors]: https://gist.github.com/kevinburke/b98e053a4bf9835c67bb
[tests]: https://github.com/kevinburke/requests/compare/retries-take-3?expand=1#diff-56c2d754173a4a158ce8f445834c8fe8R1447