Add orientation() method for curves and segments

[zbynekstara]

Sometimes, it is useful to know whether a curve is oriented clockwise or counterclockwise. This method would look at the start point, end point, and control points of the curve, and return +1 if it is clockwise, -1 if it is anticlockwise, and 0 if it is neither (it is a straight line or a wavy curve). (The signs match a convention established by the Point.cross function.)