Updated DDTeleporter

Updated DDTeleporter to have a fixed minimum chance of sending the
player to the Nether when using a dungeon exit. This also allows players
to reach the Nether without ever having been there before using a Nether
portal - that was previously impossible since DD would not "know" that
the Nether existed. Also cleaned up the code a bit and possibly fixed a
bug that would cause players to get teleported into walls.