added onSuccess callback from template helper

Basically, all we have to do is to specify a onSuccess callback like this `{{> atForm onSuccess=onSuccess}}`. The callback can looks something similar to this : 

    var closeModal = function(){
      // ...
      AccountsTemplates.avoidRedirect = true; // will prevent the redirection
      $("#modal").modal('hide');
    }
    
    Template.modal.helpers({
      onSuccess : function(){
        return closeModal;
      }
    });