function entropy( alpha, beta ) {
	var out;
	if ( alpha <= 0.0 || beta <= 0.0 ) {
		return NaN;
	}
	out = betaln( alpha, beta );
	out -= ( alpha-1.0 ) * digamma( alpha );
	out -= ( beta-1.0 ) * digamma( beta );
	out += ( alpha+beta-2.0 ) * digamma( alpha+beta );
	return out;
}
Returns the differential entropy of a beta distribution.

@param {PositiveNumber} alpha - first shape parameter
@param {PositiveNumber} beta - second shape parameter
@returns {NonPositiveNumber} differential entropy

@example
var v = entropy( 1.0, 1.0 );
// returns 0.0

@example
var v = entropy( 4.0, 12.0 );
// returns ~-0.869

@example
var v = entropy( 8.0, 2.0 );
// returns ~-0.795

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

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

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

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