code: public static <E> double jensenShannonDivergence(Counter<E> c1, Counter<E> c2) {
      Counter<E> average = average(c1, c2);
      double kl1 = klDivergence(c1, average);
      double kl2 = klDivergence(c2, average);
      return (kl1 + kl2) / 2.0;
    }
docs: Calculates the Jensen-Shannon divergence between the two counters. That is,
  it calculates 1/2 [KL(c1 || avg(c1,c2)) + KL(c2 || avg(c1,c2))] .
  
  @return The Jensen-Shannon divergence between the distributions