Use Entity.teleportTo(). Fixes BUKKIT-2471

Use Entity.teleportTo() when using Entity.teleport() to a different
world.
Entity.teleportTo() is the method used when an Entity travels through a
portal.
Previously, when the teleport location was to a different world, the
"world" field of the Entity was set to the destination world, but the
Entity was not tracked by the destination world. Entity.teleportTo()
contains pre existing code that creates a new entity in the destination
world identical to the current entity and kills the entity in the source
world while forwarding the new Entity reference to the CraftEntity.