function stdev( r, p ) {
	return addon( r, p );
}
Returns the standard deviation of a negative binomial distribution.

@private
@param {PositiveNumber} r - number of failures until experiment is stopped
@param {Probability} p - success probability
@returns {PositiveNumber} standard deviation

@example
var v = stdev( 100, 0.2 );
// returns ~44.721

@example
var v = stdev( 20, 0.5 );
// returns ~6.325

@example
var v = stdev( 10.3, 0.8 );
// returns ~1.794

@example
var v = stdev( -2, 0.5 );
// returns NaN

@example
var v = stdev( 20, 1.1 );
// returns NaN

@example
var v = stdev( 20, NaN );
// returns NaN

@example
var v = stdev( NaN, 0.5 );
// returns NaN