Support for dirty schedulers

Nifty now has experimental support for dirty schedulers. Just add schedule_dirty to your nifty options:

1> nifty:compile("long_running.h",
                 long_running,
				 nifty_utils:add_sources(["long_running"],
				                         [{nifty, [schedule_dirty]}])).

If you don’t have dirty schedulers supported in your OTP yet, then Nifty will give you a warning and compile without using them.