function kurtosis( p ) {
	return addon( p );
}
Returns the excess kurtosis of a Bernoulli distribution.

@private
@param {Probability} p - success probability
@returns {PositiveNumber} excess kurtosis

@example
var v = kurtosis( 0.1 );
// returns ~5.111

@example
var v = kurtosis( 0.5 );
// returns -2.0

@example
var v = kurtosis( 1.1 );
// returns NaN

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