Make models serializable

[jgaskins]

This can allow them to be stored in localStorage, sent to the server, etc.

It's a start to add support for #4.

[ajjahn]

Would this make sense to be an optional mixin, similar to #10?

[jgaskins]

Probably not a bad idea. It was really only ever intended to support a serializable store so that you could send your store's initial state and action history across the wire for debugging. Beyond that, I can see this being used for all the wrong reasons if it were baseline functionality.