function kurtosis( lambda ) {
	return addon( lambda );
}
Returns the excess kurtosis of an exponential distribution.

@private
@param {NonNegativeNumber} lambda - rate parameter
@returns {NonNegativeNumber} excess kurtosis

@example
var v = kurtosis( 9.0 );
// returns 6.0

@example
var v = kurtosis( 1.0 );
// returns 6.0

@example
var v = kurtosis( -0.2 );
// returns NaN

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