Makes init() public for Circle and Polygon

[traviskirton]

As a user I want to be able to create an empty Polygon, rather than an empty Shape. I want this because I can take advantage of how Polygon initializes differently than Shape.

func continuousLines() {
    let p = Path()
    let poly = Polygon()
    canvas.add(poly)
    canvas.addPanGestureRecognizer { location, translation, velocity, state in
        if state == .Began {
            p.moveToPoint(location)
        } else {
            p.addLineToPoint(location)
        }
        poly.path = p
    }
}

Same logic for Circle.