Future[T] and Promise[T] as Controller ReturnTypes

[domdorn]

This PR adds support for Future[T] and Promise[T] as return types in resources, e.g.

@Path("/")
class GreetingResource() 
    @GET
    @Path("/greet/promise")
    def promiseGreeting(): Promise[String] =
      Promise.successful("Hello from the promise")
    end promiseGreeting
end GreetingResource

I also added tests in the integration module and converted them to be in Scala