Add toSubscriber conversion

[searler]

The article uses internal implementation code from the 0.3 version, as follows:

 val sub = new Subscriber[String]() {
      ...
}
    val subSink = SubscriberSink(new RxSubscriberToRsSubscriberAdapter(sub));

This use case seems reasonable, at least in the context of integration with Akka Streams

That would require that toSubscriber methods be added to RxReactiveStreams

[ldaley]

We would add this as RxReactiveStreams.toSubscriber().