fix installation of zmq IOLoop into tornado

incorrectly overrode the module, rather than the class in the module, which is more reliable.

IPython/frontend/html/notebook/notebookapp.py

@@ -29,7 +29,7 b' import zmq'
 # tornado is imported.
 from zmq.eventloop import ioloop
 import tornado.ioloop
-tornado.ioloop = ioloop
+tornado.ioloop.IOLoop = ioloop.IOLoop
 
 from tornado import httpserver
 from tornado import web