Just added `Observable.interval()`, then need to p...

# rx

[thomasnield]

Just added Observable.interval(), then need to proof-of-concept a few combining operators. I need to explore the best way to implement Subscriber and Subscription to handle Job cancellation. Also, certain sources like Observable.just() do not need jobs until subscribeOn() or observeOn() is introduced, but operators like take() require early termination.