Add option to initialze game board
Rename #legal_move* to #possible_move*
Add board#king_position
Add Game#check?
Add Board#dupe
Rename LegitMove to PossibleMove
Add Board#move!
Mv Game#move #move!
Can't move into check
Detect checkmate
Add Game#stalemate?
Mv Board#print -> Board#to_s
Fix black pawn movement