This change adds a new integration which allows people to build prospective

changes via CircleCI. This requires the (beta) CircleCI 2.0.
This is similar in many ways to the Travis CI integration, but is somewhat

different in its configuration. The major difference is that while Travis

allows us to override the config within our API request, Circle does not. This

means that users will have to add Review Board support within their

.circleci/config.yml file.
When a config matches, this integration will trigger a build of the

"reviewboard" job (as defined in the user's config) and pass along various bits

of data in the build parameters.

Posted a variety of changes which should either fail or pass the build, and

saw the results reported correctly.