Revise V2 API serialize() methods

[toconnell]

A settlement, for example, should look like this:

{
    "meta": {<meta stuff>}, 
    "sheet": <formerly .settlement>, (to include campaign, expansions)
    "user_assets" {
        "survivors": <list of serialized survivors>,
        "players": <list of serialized users>,
     },
    "game_assets": {
       <cursed items, gear, locations, innovations, etc.>, 
     },
}

[toconnell]

Done!