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

@param {PositiveNumber} alpha - shape parameter
@param {PositiveNumber} beta - rate parameter
@returns {number} entropy

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

@example
var v = entropy( 4.0, 12.0 );
// returns ~-0.462

@example
var v = entropy( 8.0, 2.0 );
// returns ~1.723

@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