function entropy( lambda ) {
	return addon( lambda );
}
Returns the entropy of an exponential distribution.

@private
@param {NonNegativeNumber} lambda - rate parameter
@returns {number} entropy

@example
var v = entropy( 9.0 );
// returns ~-1.197

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

@example
var v = entropy( -0.2 );
// returns NaN

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