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
Unfortunately, ```GenericScheduledExecutorService``` is package private but you need the same structure and single static instance.