function kurtosis( alpha, beta ) {
	var axb;
	var amb;
	var apb;
	var out;

	if ( alpha <= 0.0 || beta <= 0.0 ) {
		return NaN;
	}
	axb = alpha * beta;
	amb = alpha - beta;
	apb = alpha + beta;
	out = amb * amb * ( apb+1.0 );
	out -= axb * ( apb+2.0 );
	out *= 6.0;
	out /= axb * ( apb+2.0 ) * ( apb+3.0 );
	return out;
}
Returns the excess kurtosis of a beta distribution.

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

@example
var v = kurtosis( 1.0, 1.0 );
// returns -1.2

@example
var v = kurtosis( 4.0, 12.0 );
// returns ~0.082

@example
var v = kurtosis( 8.0, 2.0 );
// returns ~0.49

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

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

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

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