fixes issues #4, #11, #12, and #13

[ghost]

• Issue #4: Added comments for a better understanding of the code

• Issue #11: The new Levels class holds an instance of the GameMap class and an ArrayList of GoblinFigures, allowing for the creation of different levels.

• Issue #12: The creation of the InventoryDisplay class allows the user to sort through the main character's inventory and set their current weapon. (Note: there is a strange bug where setting the SimpleRPG's frame's button to open the InventoryDisplay in either the North or West borders stops the user from being able to input keys into the Display's PlayerListener; this is why I put it in the South border)

• Issue #13: The goblin displayed onscreen will move toward the player. It is moved when the instance of the Display class is redrawn.

[extrabutter]

1. Added extensive unit testing to many classes to avoid/isolate future issues.
2. Created Trader and TraderFigure classes used within the game. The Trader appears as a white figure on the game map. MainCharacter now has to be near the Trader to be able to access the Shop. The Goblin uses ambushMove() to approach the MainCharacter while Trader uses the randMove() function for its movements.