Flatten IPython.frontend

Flatten IPython.frontend

Finishing up @fperez PR #3395, since he is unavailable. Only change is to add the final step:

    IPython.frontend.html.notebook -> IPython.html (as opposed to IPython.html.notebook)

Issued from ipython org, so any core dev can make final adjustments.

closed #3395

* [ ] announce to -dev list to remove `frontend/` directory and all stale `.pyc` files