Added proper support for multi-world Bukkit plugins such as MultiVerse!

* Fixed a few issues with world object using CB's dimension ID instead of
provider's dimension.

Fixes #292, #233, #326, #553, #617, #657, #800, #940

This was no easy task but we managed to pull it off with the help of Forge
and MultiVerse. This build brings forth many new changes with two
important changes worth noting:

- This build requires Forge 711+ on client and MultiVerse 667+.

- New worlds will no longer be created in server root. All new worlds will
  now be placed in the overworld root which is identical to Forge/FTB.

To help assist migrations, I have modified the old Forge->CB migration to
CB->Forge. All world DIMXX folders in server root will be migrated over to
overworld/DIMXX. So if you named overworld "myworld", it would be moved to
myworld/DIMXX.