0.3.1: kan.Rectangle.borderRadius added

[+] new Layer => settings.canvas can be now an HTMLCanvasElement
[+] kan.Rectangle.borderRadius attribute
[+] kan.Rectangle._setRoundedRectPath(context)
[+] kan.Rectangle._setRectPath(context)
[+] new Layer => settings.canvas can be now an HTMLCanvasElement
[~] kan.Rectangle.draw adaptations
[~] kan.Rectangle.get => kan.Rectangle.getPoint (same params)