message: optional inner radius so that Ring can be a simple Circle model
diff: @@ -4,7 +4,7 @@ namespace MakerJs.models {
  public paths: IPathMap = {};
  - constructor(outerRadius: number, innerRadius: number) {
  + constructor(outerRadius: number, innerRadius?: number) {
  var radii = {
  "Ring_outer": outerRadius,
  @@ -12,6 +12,7 @@ namespace MakerJs.models {
  };
  for (var id in radii) {
  + if (radii[id] === void 0) continue;
  this.paths[id] = new paths.Circle(point.zero(), radii[id]);
  }
  }
  