ADMIN: backports branch
As discussed on the mailing list, we should outline how we are going to do backports. Goals would be to provide a 0.19.1 bug-fix only release and at the same time allow master to proceed as 0.20/1.0 with all bug-fixes and API changes.

This issue is for how to do this, as we are ready to merge some 0.19.1 fixes. So need to put some machinery in place ASAP.
- create a 0.19.1 (or .x?) branch
- `scripts/merge-py.py` merges to master
- then cherry-pick to 0.19.1?

Some questions
- how to ensure that we are getting the appropriate commits. Do we need a comparison script?
- how do we deal with the duplicated whatsnew issue (IOW, should commiters be directed to 0.20, then we copy / fix to put _also_ in 0.19.1)?
- testing, I _think_ travis/appveyor will build both branches (master and 0.19.x)

would be nice to link some examples of how other projects do this. We have quite a lot of activity and don't want this to become a burden.
