[2.0] Tracking PR for the 2.0 changelog

This contains all of the current fixes merged into the 2.0 branch in changelog form. I'll keep updating this as new changes get merged.

*Note*: This uses [this link](https://github.com/kennethreitz/requests/compare/v1.2.3...2.0) as the source of changes. I'll keep consulting it as we work on 2.0.