function stdev( alpha, beta ) {
	return addon( alpha, beta );
}
Returns the standard deviation of a beta distribution.

@private
@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