Func0 can transparently implement java.util.concurrent.Callable.

This change doesn't change the API at all for users of `Func0`, but it makes all `Func0` objects immediately reusable with any JDK API that accepts `Callables`. For example, a `Func0` can now be submitted directly to an `ExecutorService` for asynchronous execution. It also allows the elimination of a small amount of redundant code within RxJava itself.