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