IOSSchedulers for RoboVM

Added support for IOSchedulers for RoboVM

I haven't added unit tests as 
1) they can't be automated
2) they require the simulator
It sucks, I know and I'm working on a way to automate this.

Apologies for the double PR -- had to rebranch
You need to track all submitted tasks so they can be cancelled together if the Worker is unsubscribed, therefore, you need ```CompositeSubscription```. Then you'll need to introduce a class similar to ```ScheduledAction``` which can remove itself from this composite on completion and cancel the underlying NSBlockOperation.