Add top-level IPython.nbformat API

to avoid method:module conflict, renamed convert->converter
Don't use nbformat.current in core
Don't use nbformat.current in nbconvert
move NotebookNode to top-level

should common to all nbformats

(I didn't change the old nbformats to  use it, just in case)
don't use nbformat.current in IPython.html

use top-level nbformat.read/write, v4 directly for compose
deprecation warning on nbformat.current
remove unused OuptutArea.rename_keys
fix gender on Jupyter

My French is weak.
default to NO_CONVERT in nbformat.writes
restore ability to sign v3 notebooks
fix backward `f, nb` args for nbformat.write
note about mime-bundle in nbformat doc