Add support for async Netty intents using Scala futures
deprecate the redundant Directive.success
Merge pull request #176 from daggerrz/feature/scala-futures

Add support for async Netty intents using Scala futures
spacing
General kit for response functions of future, and moving `FuturePlan`
into a `future` package.
this stack trace should be printed to stderr too
actually respond with the error
rework Futured kit so we can use the plan's configured exception handler