Update bootbox.js

Adding Ajax Functionality with the Dialog Function , Also added two events (onLoad,onAjaxLoaded)

ex:
        bootbox.dialog({
          url:"test.php",
          message: "-",
          title: "Title",
          onEscape: function() {},
          show: true,
          backdrop: true,
          closeButton: true,
          animate: true,
          className: "my-modal",
          onLoad:function( d ) {
              // Triggered when modal is shown
          },
          onAjaxLoaded:function(d){
              // Triggered when ajax request is finished
          }
});