Modified tests.cpp

Added a test case for the retreave the possible moves of a piece based
on a std::pair of a character and integer.
Modified checker_board.hpp

Added in the default constructor to deal with the committing the initial
pieces to a map of all the pieces in the initial board state.
Modified tests.cpp

Added in a test case to pull a vector of moves from the piece at a given
coordinate.

Also, added a row index to the board representation that are based on
alphabet characters.
Adding file game_piece.hpp

This will be the base class of the game pieces used in a number of
different zero sum games like checkers and chess.