add notebook signing to nbformat
sign notebooks
add nbformat.sign.NotebookNotary
add `ipython notebook trust` subcommand
use configured scheme, not stored scheme when checking signatures

protects against notebook author choosing bad hash scheme.
Notaries sign notebooks now
move `ipython notebook trust` to `ipython trust`

and move App definition to nbformat.sign (maybe it should get its own file).
tweak default profile_dir

needed for testing
test notebook signing
get_output_cell fails with no such output

rather than returning null
add wait_for_output(cell, index)

so you can wait for at least n outputs
adjustments to nb_roundtrip.js

A few waits, little changes to get it running with recent changes
update shutdown_notebook

now that get_output_cell raises if there is no such output
move signature checking to base NotebookManager

so that subclasses have less to duplicate
Python 3 renamed hashlib.algorithms to algorithms_guaranteed
add `ipython trust --reset
add notebook signing docs
Merge pull request #4824 from minrk/sign-notebooks

sign notebooks