Change Item System

Instead of having a class for each item (like Stone.java that extends
Item.java) there is now an enum where you declare the types of objects,
and Item.java now has a variable that holds what type of object it is.
Rewrite Confusing Comment

Changed the comment about MapTiles.NO_TILE from calling it an error tile
(which it isn't) to just call it a blank tile (which is more accurate)