Fix several small bugs in the notebook trust framework
Fix several small bugs in the notebook trust framework

1. The trust dialog is not called with the notebook context, so we have to use the closure to bind to a notebook event.

2. Replace != with !==

3. The trust_changed.Notebook event default handler expects the trust value to be value key inside an object.  In fact, the event handler is redundant with the code--both seem to set the trust value of the current notebook.

ipython/ipython,ipython/ipython