def center (self):
    """center() -> (x, y)

    Returns the center (of mass) point of this Polygon.

    See http://en.wikipedia.org/wiki/Polygon

    Examples:

    >>> p = Polygon()
    >>> p.vertices = [ Point(3, 8), Point(6, 4), Point(0, 3) ]
    >>> p.center()
    Point(2.89285714286, 4.82142857143)
    """
    Cx    = 0.0
    Cy    = 0.0
    denom = 6.0 * self.area()

    for segment in self.segments():
      x   = (segment.p.x + segment.q.x)
      y   = (segment.p.y + segment.q.y)
      xy  = (segment.p.x * segment.q.y) - (segment.q.x * segment.p.y)
      Cx += (x * xy)
      Cy += (y * xy)

    Cx /= denom
    Cy /= denom

    return Point(Cx, Cy)