reduce a few implicit imports

- starting the notebook no longer implies IPython.parallel until clusters are started
- IPython.parallel no longer implies numpy (or numeric or numarray)
- move zmq.ssh, which implies paramiko, into local imports when it's actually used
We could check if numpy is in sys.modules before loading it, so that we don't load it unnecessarily. If it's not in use, that would mean an extra dictionary lookup each time this is called, though. I don't feel strongly about it (since I don't do parallel stuff anyway)