update ClassLimitManager to provide a set of names of Players who have picked the provided ArenaClass. This will allow manipulating players of just a certain class more easily than before. One possibility is (mind you this is NOT implemented) adding potion effects to a class
Merge pull request #194 from acstache/bleeding

Keep track of which players picked which class

- Change MutableInt counters to String sets of player names
- Add getPlayersWithClass() method