Partially implement the vanilla gamerules.

- Implemented: reducedDebugInfo,
- TODO
- Fixed GlowWorld.setGameRuleValue to not check for existing rules,
  which is a behavior change to Bukkit, but matches vanilla in
  allowing custom gamerules, which are possibly useful for command
  blocks. Once the Bukkit forking situation is settled I'll fix
  whichever Bukkit Glowstone uses to match vanilla.