Bukkit

Allow setting the spawn world to none, to avoid conflicts with other plugins. Note: if a player is in an instance which gets deleted, they will be teleported to the first world's spawn point if there is no spawn world set.
Added support for setting gamemode, based on a third party worldmanager's gamemode setting for the world. Multiverse is the only supported world manager.
Rewrote instance loading code, no longer requires destination world to be loaded.
Instances no longer always spawn monsters, this setting should be correctly cloned from the destination world.
Instances have their own chunk generator which only generates empty chunks.
Fixed bug with instance unloading happening whilst players are in the instance.
Fixed bug with instance unloading never happening in some cases.
Instance difficulty is now a setting on the portalpair, defaults to difficulty of the destination world at creation time.
Optimized onPlayerMove to only perform checks when the player has crossed a block boundary.

Chunks for instances are stored entirely in memory.