Improve 'saving' strategy

[cdriesler]

The current 'session restore' strategy just saves and then sends back the list of elements in the graph. With the introduction of the registry in #42 , this strategy has to include a bit more data.

Instead of just duct-taping the registry object to the current setup, take a moment to step through the saving logic and make it easier to add more info in the future. (i.e. a 'version' number so the graph knows not to try and read outdated graphs)