Creating Observables in Scala: Approach04

This implements **approach04** from https://github.com/Netflix/RxJava/issues/549 .
I also moved the `toScalaXxx`/`toJavaXxx` methods into a separate file, so that one can import only these without importing the implicit function conversions. 
There's now also `Observable.from(Future)` and to test it, some tweaks in `Notification` were necessary.
Note that this is a breaking change.