Skeleton of checkers.rb
Moved Board to new file; set up methods to rewire board#pos and board#pos=
Renamed checkers.rb to piece.rb
Adds Piece#moves to calculate the available moves for a piece; adds Board#on_board? to determine if a position is on the board
Adds board#setup_board to setup the pieces on initialize
Piece#moves returns only valid moves on the board. Adds images for the pieces
Moves perform_jump and perform_slide to piece.rb. Removes jumped piece when performing jump
Changes piece#perform_slide and piece#perform_jump so that you only need to enter end_pos
Adds piece#perform_moves!
Adds game.rb.  Game runs.  Error handling not yet implement
Adds lots of errors and error handling for playing the game
Fixes piece#maybe_promote
changing commits