old_contents: new Heidelberg($('#Heidelberg-example-1'), {
    previousButton: $('#previous'),
    nextButton: $('#next'),
    onPageTurn: function(el, els) {
    }
  });
  
  new Heidelberg($('#Heidelberg-example-2'), {
    canClose: true,
    arrowKeys: false,
    concurrentAnimations: 5
  });
  
new_contents: new Heidelberg($('#Heidelberg-example-1'), {
    previousButton: $('#previous'),
    nextButton: $('#next'),
    onPageTurn: function(el, els) {
      console.log('Page turned');
    },
    onSpreadSetup: function(el) {
      console.log('Spread setup');
    }
  });
  
  new Heidelberg($('#Heidelberg-example-2'), {
    canClose: true,
    arrowKeys: false,
    concurrentAnimations: 5
  });
  
  var progressIndicators = function(page, el){
    var indicators = el.parents('.book-module').find('.book-module-indicator');
    indicators.removeClass('filled');
    indicators.slice(0, page).addClass('filled');
  };
  
subject: Add callback examples