Updated Piece.java and Board.java

Piece.java -- Added private variables currentTile and type
** Unsure of whether type is necessary. Should that variable instead
belong to the individual classes for each type of piece?
** How should the variable that contains the image be implemented?
Board.java -- Added private variables and member methods
** Do we need more variables?
** Added a "getTile", "getPieceAtLocation", and "getNumSquares" method.
** Added a addTile method. Updates the tile array of a board object and
also updates the x and y position of the Tile.