Adds Piece#legal_slides
Fixes typo
Refactors Piece#legal_slides.
Adds Piece#legal_jumps.
Adds [] and []= methods in Board.
Updates Board#display.
Cleans up Piece#change_position
Adds king promotion
Fixes typo in Piece#maybe_promote
Updates test.rb and starts piece#perform_moves method
Updates test.rb
Adds valid_move_sequence method
Refactors piece.
Adds Game class
Adds Player class
Adds README