SpawnerCreature player-based mob caps

This is to address SMP mob spawning issues as shown here: http://www.youtube.com/watch?v=q_DwPJZd2BU

This is a change from the way vanilla works. Instead of having a dimension-wide mob cap for all players in the dimension, have a local cap per player.

The result should be that instead of mobs clumping around certain players, they are better distributed between the players.