function logistic( rand, mu, s ) {
	var u = rand();
	return mu + ( s*ln( u / (1.0-u) ) );
}
Returns pseudorandom number drawn from a logistic distribution.

@private
@param {PRNG} rand - PRNG for uniformly distributed numbers
@param {number} mu - mean
@param {PositiveNumber} s - scale parameter
@returns {number} pseudorandom number