Initial commit
Add .gitignore file
Create the Piece abstract class
Create PieceType

This class contains all the constants defining the type of the piece:
king, queen, bishop, knight, rook and pawn.
Create the King class
Create the Queen class
Create the Rook class
Create the Bishop class
Create the Knight class
Create the Pawn class
Create the Square class
Create the Board class, representing the chess board
Create the Color class
Create index.php, which manages the game initialization
Add isEmpty() method to Square

This allows us to know if there is any piece on the square
Add getters and setters for the piece attribute in Square
Create the Position class
Create the Position class
Create the PositionChecker class, which checks if a position is valid
Add a getSquare() method to Board to retrieve a square at defined position