Added locate_king and checked?
Add Board#move function and fix bug with Board#checked?
Add functionality to invalidate moves into check

This includes redefining the old Piece#valid_moves function as unblocked_moves,
writing a new valid_moves function, writing a Piece#move_into_check? function,
and writing a Board#clone function.
Added checkmate
Added game and player classes. Game now playable.
Colorize board display and add chess icons
Remove test file
Hardened user input and minor UI tweaks
Added notation to board display.
Refactored piece offsets.
Added win sequence
Refactored a few places.

Moved notation to game function rather than constants on multiple classes.

Monkey patched the brackets method ([]) for the board for convenience.

Refactored the pawn's valid_moves function
.
Refactored piece classes to keep some methods private
Fix rare bug with pawn movement

In some cases, a pawn could lose its ability
to double move if a piece had previously moved
directly in front of it.
Made helper functions private.

Fixed pawn double move bug.
Add stalemate condition