Add toObservable() to the common HystrixExecutable interface

All of the implementers of HystrixExecutable<R> implement toObservable() and toObservable(Scheduler observeOn)

This patch adds those methods to the common interface, and also adds @Override annotations to appropriate places.

Use case is that I have a factory method that may return a HystrixCollapser or a HystrixCommand, and want to be able to call .toObservable on the result. (I'm still doing some designs on this and may not use it that way, but hopefully this technically-breaking change can make 0.40....)