Only provide an asynchronous, Future-based API

I love Monads 'n' stuff, but we don't need them in the Content API client anymore. If a user really wants a synchronous client (and why would they, in Scala?), then they can just use `Await.result`.

I know this branch is confusingly called 'add-a-circuit-breaker', but it's more a preliminary clean up so that we can do that.