Interval

I have attempted to implement the "Interval" operator (#55) here. As far as I can see, it seems to work. This is based on the quite fresh work on schedulers.

It doesn't work with the `currentThread` or `newThread` schedulers, I guess due to the simple `SleepingAction`. It does work with a `ScheduledExecutorService`, though.

For testing this conveniently, I also wrote a test scheduler with adjustable time.

Looking forward to any review comments.