This constructor is used to create a movement command.
commandStr - The direction the player is attempting to move in.
public java.lang.String execute()
This method executes a movement command, moving the adventurer from one room to another in a specific direction. If the exit the player is trying to move through is locked, it returns a string indicating that they must unlock the exit first.
The description of the new room that the user moved to, or a string indicating that the exit they are trying to move through is locked. Conversely if there is no exit in that direction it will return a string saying they can't go that direction.