showChapter(background, duration) {
    if (duration == null) { duration = 0.0; }

    this.stopShowingChapter(duration);

    // create scene elements
    this._bg = BaseSprite.create(background);
    this.addChild(this._bg);

    // resize once to layout all elements
    this.onResize();

    // TODO: animate elements in
    return Promise.resolve();
  }
Starts showing a chapter and returns a promise.
@param {String} background path to background asset
@param {Number} [duration=0.0] duration to show over
@returns {Promise}