function stdev( a, b ) {
	var m1;
	var m2;
	if (
		isnan( a ) ||
		a <= 0.0 ||
		isnan( b ) ||
		b <= 0.0
	) {
		return NaN;
	}
	m1 = b * beta( 1.0 + ( 1.0/a ), b );
	m2 = b * beta( 1.0 + ( 2.0/a ), b );
	return sqrt( m2 - ( m1*m1 ) );
}
Returns the standard deviation of a Kumaraswamy's double bounded distribution.

@param {PositiveNumber} a - first shape parameter
@param {PositiveNumber} b - second shape parameter
@returns {PositiveNumber} standard deviation

@example
var v = stdev( 0.5, 1.0 );
// returns ~0.298

@example
var v = stdev( 4.0, 12.0 );
// returns ~0.13

@example
var v = stdev( 12.0, 2.0 );
// returns ~0.077

@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