Use rx.Single

[mattrjacobs]

As of RxJava 1.1.0, rx.Single is now available. This is a scalar async value, very similar to a j.u.c.CompletableFuture, but with the Rx operators.
#602 will introduce the concept of a scalar and async command

There is also the possibility of adding a method to HystrixCommand to get a Single out. Or we could even discuss breaking backwards-compatibility and returning an rx.Single from observe() and toObservable(). In terms of types, it's likely the most accurate type for those operations.

Marked as 'discussion' as I'd appreciate feedback on if the above would be valuable.