Add support for CircleCI

CircleCI gives us faster, more reliable builds than Travis, since it has a
higher memory limit (4G vs 2G) and also has Docker running natively, rather
than us having to hack it into a usermode Linux instance.

* Add circle.yml configuration.

* Add a circle build profile.

* Add circle.sh to make and run socat. We should talk to the Circle guys to
  see if they can expose Docker via TCP so we don't have to do this.