Add unit & integration testing, tied in to Travis

I'd written these already for our fork, but now that some prereqs have merged in (flinc-chef/motd#3, flinc-chef/motd#5, flinc-chef/motd#6), this can come in cleanly.

I've rebased it based on the current state of flinc-chef/motd@master, so it wraps up the existing linting tests & couples them to unit tests (on Travis) and integration tests (when run locally).