Add continuous integration tests and auto-merge

[dnadlinger]

We should have CI tests for

• Python unittests,
• code style (YAPF), and
• Flake8,

and a bot to auto-merge PRs on passing tests.

Any of the usual suspects (Travis, Semaphore, …) will do; the only aspect that requires a modicum of thought is handling the ARTIQ dependency.

[dnadlinger]

We now have all the above; hosted on CircleCI for its straightforward Docker integration.