function digamma( x ) {
	var y;
	var z;
	x -= 1.0;
	y = ln(x) + ( 1.0 / (2.0*x) );
	z = 1.0 / (x*x);
	return y - ( z*polyval( z ) );
}
Evaluates the digamma function via asymptotic expansion.

@private
@param {number} x - input value
@returns {number} function value