function stdev( alpha, beta ) {
	var apb;
	var out;
	if ( alpha <= 0.0 || beta <= 0.0 ) {
		return NaN;
	}
	apb = alpha + beta;
	out = sqrt( ( alpha * beta ) / ( apb + 1.0 ) );
	out /= apb;
	return out;
}
Returns the standard deviation of a beta distribution.

@param {PositiveNumber} alpha - first shape parameter
@param {PositiveNumber} beta - second shape parameter
@returns {PositiveNumber} standard deviation

@example
var v = stdev( 1.0, 1.0 );
// returns ~0.289

@example
var v = stdev( 4.0, 12.0 );
// returns ~0.105

@example
var v = stdev( 8.0, 2.0 );
// returns ~0.121

@example
var v = stdev( 1.0, -0.1 );
// returns NaN

@example
var v = stdev( -0.1, 1.0 );
// returns NaN

@example
var v = stdev( 2.0, NaN );
// returns NaN

@example
var v = stdev( NaN, 2.0 );
// returns NaN