Support for different types of UseCases

[RiccardoM]

As of RxJava 2.0 new types have been added, such as Maybe, and also many more specific ones are already implemented in RxJava 1.x, such as Single and Completable.
I think that adding few abstract UseCase classes that let introduce this kinds of return types would be great, letting us creating more specific use cases (e.g. If I need just a string there's no need to return an Observable object). Some examples could be MaybeUseCase or SingleUseCase, and so on.