Get rid of nose, and ref to IPython.testing.

[Carreau]

For next major of IPython I'm going to try to nuke everything related to nose, that include a lot of thing in IPython.testing.

I suggest IPykernel try to do the same as nose has been unmaintained for quite some time, and that any reference to IPython.testing be check to see if it can be replaced.

[Kojoley]

I think this could be closed now. IPython.testing is no longer used by ipykernel and the last nose imports are optional (IPython compatibility thing). The #397 is no longer relevant too.

[Carreau]

Yep, thanks, let's close.