Adds skeleton of Checkers class. Fixes Board#initialize and adds Board#fill_rows helper to fill empty grid.
Adds Checkers#initialize. Adds Checkers#parse_input.
Fixes Checkers#parse_input, Checkers#get_pos. Adds Checkers#get_move_sequence / Checkers#render.
Fixes bug in Board#initialize / typo in Board#pieces. Adds Checkers#turn.
Adds Checkers#play. Fixes typo in Checkers run as script.