Rename BiomeManager to TCWorldChunkManager

So that the Bukkit and Forge names are the same.
Fix #25 by reporting the correct sea level

- On Forge this is done by overriding a method in WorldType.
- On Bukkit, which doesn't use the world type system for custom
generators, it is done by overriding WorldProvider.
WorldConfig improvements

- MinimumDistanceBetweenRareBuildings is now accurate
- updated objectSpawnRatio description - I forgot to update it when I
changed the setting for 2.4.3.