Add "id" back to NotebookSession class

[mpacer]

Currently we are consuming the id passed through via the sessions response http://jupyter-api.surge.sh/#!/sessions/get_api_sessions_session in the nb_client library and not setting it to a variable. My guess is that this was removed earlier because of similar issues to those raised around keeping the kernel's "id" field.

I discovered this as I was writing unit tests for the client section; I am going to refrain from adding it in those tests, but will leave commented code in place to indicate the functionality's future return (mostly for the sake of future integration tests).