Fix parsing errors in parse_algebraic_notation
Remove old debug messages
Begin computer player
Add notes on computer AI to readme
Refactor, add player class, fix turn-taking bugs in chess.rb
Add enemy? method
Add pawn promotion
Add castling
Minor refactoring
Split HumanPlayer out into its own file
Require human_player
Start computer AI, more castling
Remove debug messages
Add draw conditions and computer player pawn promotion
Computer player looks for checks