o Added a heatbugs-like JavaScript model

I don't know if you're interested in this.  But the sketches/jsmodel.html didn't go quite far enough for me.  My playing around lead me to this model and I thought it might be helpful for others.
Do you think you could throw in some comments here to explain what's going on in step? Maybe after each conditional statement? For instance, I think it would be good to explain the meanings of `happiness`, `unhappiness`, and `pheromone`.