That's why I question whether we should support `Executor` for this as it doesn't match the use cases of `Scheduler` (it's too simple). Why not just have options such as:

- Schedulers.threadPoolForComputation() - core size set to number of CPU cores
 - similar to `send` in clojure (http://clojure.github.com/clojure/clojure.core-api.html#clojure.core/send)
- Schedulers.threadPoolForIO() - pool allowed to grow (to some high max? unlimited?)
 - similar to `send-off` in clojure (http://clojure.github.com/clojure/clojure.core-api.html#clojure.core/send-off)
- Schedulers.fromScheduledExecutorService(ScheduledExecutorService impl)
 - if someone wants to inject their own implementation

The `Schedulers.threadPoolForComputation()` would in effect be a multithreaded event-loop shared across the system. Hystrix would for example use it on https://github.com/Netflix/Hystrix/issues/123 to perform the callbacks.