don’t import IPython.parallel until it’s used

in IPython.html.notebookapp
avoid unnecessary imports of numpy

in IPython.parallel.map

also drop crazy-old numeric / numarray support
delay imports of zmq.ssh.tunnel

until they are actually used.

Prevents unnecessary imports of Paramiko
Merge pull request #6042 from minrk/import-less-than-everything

reduce a few implicit imports