for downloads, replaced window.open with window.location.assign

This doesn't actually open a new windows, so it's a bit nicer. 
Not very important, though.
Source: [StackOverflow](http://stackoverflow.com/questions/1066452/easiest-way-to-open-a-download-window-without-navigating-away-from-the-page)

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

@@ -58,13 +58,13 b' var IPython = (function (IPython) {'
             var notebook_id = IPython.notebook.get_notebook_id();
             var url = $('body').data('baseProjectUrl') + 'notebooks/' +
                       notebook_id + '?format=json';
-            window.open(url,'_newtab');
+            window.location.assign(url);
         });
         this.element.find('#download_py').click(function () {
             var notebook_id = IPython.notebook.get_notebook_id();
             var url = $('body').data('baseProjectUrl') + 'notebooks/' +
                       notebook_id + '?format=py';
-            window.open(url,'_newtab');
+            window.location.assign(url);
         });
         this.element.find('button#print_notebook').click(function () {
             IPython.print_widget.print_notebook();