Refactor: text features of stones moved to Goban

Now Stone class only cares about "color" as a integer value.
Goban hides the ugly text conversions since it does most of the console
rendering (debug & console game).
Errno::EAGAIN now expected by web server (thanks Pierre for info).
Inverted O and @ for black and white stones representation.
Inverted O and @ for black and white stones representation.

Updated after Goban/Stone refactor.
Inverted O and @ to follow goban change
Made 2 classes for empty zones analysis

Currently in util.rb; will move on with more changes