function kurtosis( p ) {
	if (
		isnan( p ) ||
		p <= 0.0 ||
		p >= 1.0
	) {
		return NaN;
	}
	return 6.0 + ( ( p*p ) / ( 1.0-p ) );
}
Returns the excess kurtosis of a geometric distribution.

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

@example
var v = kurtosis( 0.1 );
// returns ~6.011

@example
var v = kurtosis( 0.5 );
// returns 6.5

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

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