The spaceship you are going to add is just another sprite, but you are going to give it behavior like it was ruled by gravity.

First, let' s add a couple of variables:

var background;
var gameLayer;
var scrollSpeed = 1;
var ship;
var gameGravity = -0.05;

The ship variable will be the spaceship itself, whereas gameGravity is the force that will attract the spaceship toward the bottom of the screen.

Then, inside the init function in game declaration, you add the ship in the same way you added the background:

var game = cc.Layer.extend({
  init:function () {
    this._super();
    background = new ScrollingBG();
    this.addChild(background);
    this.scheduleUpdate();
    ship = new Ship();
    this.addChild(ship);
  },
  update:function(dt){
    background.scroll();
    ship.updateY();
  }
});

Then, in the update function (remember this function is automatically called at each frame). Thanks to the scheduleUpdate method, an updateY custom method is called.

The creation of the...