function skewness( alpha, beta ) {
	var out;
	var ab;
	if ( alpha <= 0.0 || beta <= 0.0 ) {
		return NaN;
	}
	ab = alpha + beta;
	out = 2.0 * ( beta-alpha ) * sqrt( ab + 1.0 );
	out /= ( ab + 2.0 ) * sqrt( alpha * beta );
	return out;
}
Returns the skewness of a beta distribution.

@param {PositiveNumber} alpha - first shape parameter
@param {PositiveNumber} beta - second shape parameter
@returns {number} skewness

@example
var v = skewness( 1.0, 1.0 );
// returns 0.0

@example
var v = skewness( 4.0, 12.0 );
// returns ~0.529

@example
var v = skewness( 8.0, 2.0 );
// returns ~-0.829

@example
var v = skewness( 1.0, -0.1 );
// returns NaN

@example
var v = skewness( -0.1, 1.0 );
// returns NaN

@example
var v = skewness( 2.0, NaN );
// returns NaN

@example
var v = skewness( NaN, 2.0 );
// returns NaN