Add support for servers to register dimensions on client.

In order to support multi worlds such as MultiVerse, the server must be
allowed to register multi world dimensions on client or many forge mods
such as IC2 will not function correctly. This is currently a major issue
with servers such as MCPC which provider both Forge and Bukkit support to
players. By adding the DimensionRegisterPacket class, the server now has the ability to send a packet to client with the required information to register a dimension in DimensionManager.