Backport PR #322: require tornado

we use zmq.eventloop, which generally requires tornado. The polyfill allowing this to be importable without tornado present is going away in a future release of pyzmq.

closes  321

Signed-off-by: Thomas Kluyver <[email protected]>

Author: takluyver

setup.py

@@ -82,6 +82,7 @@ def run(self):
         'jupyter_core',
         'pyzmq>=13',
         'python-dateutil>=2.1',
+        'tornado>=4.1',
     ],
     extras_require   = {
         'test': ['ipykernel', 'ipython', 'mock'],