Basic queues plugin

Unfortunately this "global list of things" that exist in an API instance is a bit of an anti-pattern.  Given that literally all the other plugins do it this way right now, I can't really blame you for copying the design.  In the future we should be keeping these types of things as attributes on a user's `Session` object, rather than globally.  However, in the meanwhile, rather than creating a new `canned_responses` submodule, would you consider putting this `queue_cache` dictionary as an attribute on `QueueApi` so that at least it's isolated in the tests?