Session model spec

A session is essentially the notebook model loaded plus some meta information.

Field	Type	Example(s)
id	String	xYf2s5
notebookModel	JSON	{}
title	String	MyNotebook
path	String	/User/PayneDiaz/myNotebook.bkr
pathType	String	file, http
fileType	String	bkr, ipynb
openDate	long(millis)	499162800000
edited	Boolean	true