CI: add circle ci support
this adds support for using CircleCI; configured to put 4 of our builds (3.4, 3.4-slow, 2.7 compat, and 3.5 ascii), they are still on Travis ATM. They are built/tested simultaneously on CircleCI (as we get 4 containers for open-source).

CircleCI is a PITA. It has parallelism, but no build-matrix. So this took a while to get right. Don't currently use pytest-xdist as was getting some failures, but maybe can turn on at some point.

I'll come back later and remove the builds from TravisCI when everything's all connected.