Prepare new release (2.1.0)?

There are a few nasty bugs that we've fixed since the last release, including the broken SNI support. It'd be good to get another release out. I think a few things have changed dramatically enough that this is probably a minor release (e.g. takes us to 2.1.0). Thoughts? /cc @kennethreitz @sigmavirus24

Still to do:

- [ ] Merge outstanding PRs (definitely #1713, #1657; maybe #1729, #1628 and #1743 depending on how @kennethreitz feels).
- [ ] Update changelog to match those PRs.

If nothing else Runscope could do with this release, so it'd be nice to be in good shape.