Created GameState

Created the class designed to represent the state of the game at any
given moment.
Removed Herobrine.

Todo: Finish setPieceAt method in GameState.
Todo: Add setX and setY method in Piece.
Todo: Finish documenting all currently existing classes before
progressing.
Todo: Replace all instances of {List<List<Piece>> board} with
{GameState board} in Piece.java and Pawn.java.
Todo: Figure out how this whole thing is supposed to work together
anyway.
Todo: Remove spaghetti from code.
Todo: Remove Herobrine.