Adds a find_middle method to checkerboard.rb

find_middle finds a coordinate between a specified starting coordinate and a
specified ending coordinate. This is useful for finding the piece between a
piece to move and the square to which it intends to jump.
perform_jump method now checks if a piece is jumped.

Currently bugged and does not remove the jumped piece.
Adds a render method to Board to return string representation.

Also adds a to_s method to Piece for this purpose.
Fixes bug in find_middle and perform_jump

find_middle could previously modify other position arrays, leading to
unpredictable results when used in perform_jump. perform_jump now works as
intended.