Optimized Weighted Random Dungeon Selection

Optimized the selection of random dungeons with weights applied. We now
have a class called WeightedContainer for taping into Minecraft's
weighted selection code without having to extend the WeightedRandomItem
class. Using that, we no longer need to keep a list with duplicate
dungeons to achieve weighted selection, so I removed that variable.
Merge remote-tracking branch 'upstream/master' into R1.4.0-improvements
Merge pull request #27 from SenseiKiwi/master

Optimized Weighted Random Dungeon Selection