Added NextWorld and ChangeToNextWorld

NextWorld returns the next world after the player-given world.
ChangeToNextWorld logs the player into the next non-full world after the
player-given world.
Updated ChangeToNextWorld

Updated ChangeToNextWorld so that it returns the world it changed to
instead of whether or not it changed worlds (since it will always change
worlds).
Fixed a space in an output statement.

OCD haha.
Merge pull request #126 from Leespiker/master

Added NextWorld and ChangeToNextWorld