Add support for servers to register dimensions on client.

In order to support multi-worlds such as MultiVerse, the server needs the
ability to register dimensions on client or many forge mods such as IC2
will not function correctly. This has been an issue for MCPC which
provides both Forge and Bukkit support to players. By adding the
DimensionRegisterPacket class, MCPC now has the ability to send the
required packet to client to register a dimension with DimensionManager.