Wrapped rollback with a Callable to make sure that it executes on the Bukkit thread.

Saw this in a log:

java.util.ConcurrentModificationException
	at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:819)
	at java.util.ArrayList$Itr.next(ArrayList.java:791)
	at me.StevenLawson.TotalFreedomMod.TFM_RollbackManager.rollback(TFM_RollbackManager.java:94)
	at me.StevenLawson.TotalFreedomMod.Commands.Command_gtfo.run(Command_gtfo.java:51)