Add facilities for creating Observables from JavaFX events and ObservableValues

Heavily inspired by the swing contrib.

Provides:
 * a Scheduler implementation ensuring execution in the JavaFX UI thread
 * a Subscription implementation ensuring that un-subscription happens in JavaFX UI thread
 * an Observable factory method for observing UI events on JavaFX nodes
 * an Observable factory method for observing value changes of a JavaFX ObservableValue

Also the rxjava-javafx is added to the gradle build.