Unintuitive API for trust/signing a notebook

I was working on [this issue](https://github.com/jupyter/notebook/issues/195) in the notebook repo and was confused by the ``NotebookNotary`` class's API.

The ``check_cells`` method returns a boolean, but it also pops off the ``trusted`` key from cells' metadata. IMO, you can never tell, looking at the API, that it mutates the notebook contents model passed to it. Also, if you do not use check_cells and call ``sign`` or ``compute_signature`` or ``check_signature`` directly, the ``trusted`` key is not popped off.

Could we not mutate the notebook model [here](https://github.com/jupyter/nbformat/blob/master/nbformat/sign.py#L484) ? And, it should be conveyed to the user that ``sign`` and other like methods would require you to remove the ``trusted`` key.

If you agree something should be done here, I'd be willing to work on it.

Thanks!