alphabetized methods in classes
minor re-factoring of slide/jump
refactored place_pieces
Board#dup added
pieces can perform sequences of moves, checks validity
setting up game class
possibly promotes to king after each move
added board end game logic
fixed promotion/perform moves mix-up bugginess
set-up play loop and improved UI
displays 'king' symbols
refactored input loop
fixed king moves bug, added 'kingness' to dup method