add tooltip to pager button

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

@@ -48,6 +48,7 b' var IPython = (function (IPython) {'
         var that = this;
         this.pager_button_area.append(
             $('<a>').attr('role', "button")
+                    .attr('title',"open the pager in an external window")
                     .addClass('ui-button')
                     .click(function(){that.detach()})
                     .attr('style','position: absolute; right: 10px;')