Clear window title when kernel is restarted

When kernel is died and restarted, or restarted while it is in the
busy state, message "(Busy)" on the window title is not updated.  This
problem is fixed by updating document title when restarting.

IPython/frontend/html/notebook/static/js/notificationwidget.js

@@ -48,6 +48,7 var IPython = (function (IPython) {
             that.set_message("Kernel busy");
         });
         $([IPython.events]).on('status_restarting.Kernel',function () {
+            IPython.save_widget.update_document_title();
             that.set_message("Restarting kernel",500);
         });
         $([IPython.events]).on('status_interrupting.Kernel',function () {
@@ -64,6 +65,7 var IPython = (function (IPython) {
                 title: "Dead kernel",
                 buttons : {
                     "Restart": function () {
+                        IPython.save_widget.update_document_title();
                         that.set_message("Restarting kernel",500);
                         IPython.notebook.start_kernel();
                         $(this).dialog('close');