code: function(a,b,d){
  	var o, dir;
  	if(arguments.length == 3){
  		Vec2D.apply(this,[a,b]);
  		this.dir = d.getNormalized();
  	} else if(arguments.length == 2){
  		Vec2D.apply(this,[a]);
  		this.dir = b.getNormalized();
  	} else if(arguments.length === 0){
  		Vec2D.apply(this);
  		this.dir = Vec2D.Y_AXIS.copy();
  	}
  }
docs: Ray2D accepts 2 formats for its constructor
  Option 1:
  @param {Number} x,
  @param {Number} y,
  @param {toxi.geom.Vec2D} direction
  
  Option 2:
  @param {toxi.geom.Vec2D} position
  @param {toxi.geom.Vec2D} direction