Match scoped Metadatable support

[Electroid]

Bukkit has a Metadatable interface that allows for various things to have metadata. Right now, to create a FixedMetadataValue or LazyMetadataValue you need to access PGM.get() -- this is not a good pattern. Plus, nothing prevents these values from leaking across matches.

Similar to what we have done with Scheduler, there should be a "helper" class that assists in setting and getting metadata values. It can be scoped to a MatchScope and is removed when the Match leaves that scope.