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
Also, you may want to check if ```isUnsubscribed()``` at this point eagerly to prevent scheduling on a terminated  worker.