function entropy( p ) {
	return addon( p );
}
Returns the entropy of a Bernoulli distribution.

@private
@param {Probability} p - success probability
@returns {PositiveNumber} entropy

@example
var v = entropy( 0.1 );
// returns ~0.325

@example
var v = entropy( 0.5 );
// returns ~0.693

@example
var v = entropy( 0.9 );
// returns ~0.325

@example
var v = entropy( 1.1 );
// returns NaN

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