function kurtosis( alpha, beta ) {
	return addon( alpha, beta );
}
Returns the excess kurtosis of a beta distribution.

@private
@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