Modified game_piece.hpp

Made a Checker_Piece class that inherits from the Game_Piece class.
Reformatted the constructor to only take a character that would be 'r'
or 'w' with the case indicating the type of the checker piece.
Modified checker_board.hpp

Changed the map and initialization of the board to use the new
construction of the Checker_Piece class.
Modified tests.cpp

Added a test case for the checker moves. Simple testing constructed red
and white pieces in the red corner (a0) of the board.
Merge branch 'checker-pieces' into checker-board

Merging the updated Checker_Piece class in to the board branch.
Modified checker_board.hpp

Added a loop in the get_moves() function to delete moves of a piece if
there is a piece already there.