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

@private
@param {NonNegativeNumber} lambda - rate parameter
@returns {number} skewness

@example
var v = skewness( 9.0 );
// returns 2.0

@example
var v = skewness( 1.0 );
// returns 2.0

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

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