Return Comm rather than BaseComm from create_comm

[matthewturk]

trafficstars

This replaces BaseComm in create_comm with Comm, which should match what is expected by ipywidgets for validation of the comm trait in subclasses of ipywidgets.Widget.

Fixes #1090 .

[matthewturk]

It looks like the failure is in ipyparallel with:

______________________ AsyncResultTest.test_wait_for_send ______________________
  
  self = <ipyparallel.tests.test_asyncresult.AsyncResultTest testMethod=test_wait_for_send>
  
      def test_wait_for_send(self):
          view = self.client[-1]
          view.track = True
  >       with self.assertRaises(TimeoutError):
  E       AssertionError: TimeoutError not raised
  
  ipyparallel/tests/test_asyncresult.py:377: AssertionError

My intuition doesn't have much to say about if this is related.

[blink1073]

It seems to be an intermittent failure. I saw it in #1089, but then it passed on the merge commit. I just kicked it.

[blink1073]

cc @maartenbreddels @martinRenou