function entropy( v ) {
	return addon( v );
}
Returns the differential entropy of a Student's t distribution.

@private
@param {PositiveNumber} v - degrees of freedom
@returns {number} entropy

@example
var v = entropy( 9.0 );
// returns ~1.533

@example
var v = entropy( 2.0 );
// returns ~1.96

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

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