Set css display to "" on notification close

When the notification is hidden (via boostrap-alert), a style of display:none is added. By removing this styling, the same notification object can be re-used.

ie. you can do:

    var _notif = $('.elem').notify();
    _notif.show();
    // wait some time for it to fade out, then...
    _notif.show() // this will display again; previously nothing would be visible due to the display:none