Backfill test coverage

[kench]

Test Tip of the Day: If the code deserves to be written, it deserves to have tests.

There should be some test cases that cover the end-to-end interactions with Bandshell. Whether that's done with unit tests and mocks, integration tests with a demo (or the RPI) instance, or a fake client calling Bandshell directly.

Having automated test cases allows others to understand what the various pieces of Bandshell are doing too!

I've already set up CircleCI for the repository. All it does this moment is perform a sanity test and verifies that the resulting RubyGem can be installed - a start, but a low bar in terms of automated testing.