function entropy( p ) {
	var q;
	if (
		isnan( p ) ||
		p <= 0.0 ||
		p >= 1.0
	) {
		return NaN;
	}
	q = 1.0 - p;
	return ( -( p*ln(p) ) - ( q*ln(q) ) ) / p;
}
Returns the entropy of a geometric distribution.

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

@example
var v = entropy( 0.1 );
// returns ~3.251

@example
var v = entropy( 0.5 );
// returns ~1.386

@example
var v = entropy( 0.9 );
// returns ~0.361

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

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