Scalar Asynchronous HystrixCommand

[mattrjacobs]

See #315 for history.

We already have HystrixCommand<T>, which is synchronous and scalar, and HystrixObservableCommand<T>, which is asynchronous and streaming.

We should add an asynchronous, scalar command as well.

Possible names:

• HystrixFutureCommand
• HystrixAsyncCommand

This will rely on a (as-yet-undefined) scalar rx.Observable, tracked here: https://github.com/ReactiveX/RxJava/issues/1594

[mattrjacobs]

Also see #321

[mattrjacobs]

RxJava 1.0.13 now includes rx.Single, so this work should be unblocked

[brunoalmeida-fairtiq]

What about CompletableFuture or a CompletionStage? That way, you can just base this on the universal Java API