New Serialized Format

Serialized graphs now look like this:

{
  "nodes": [
    { "id": "a" },
    { "id": "b" },
    { "id": "c" }
  ],
  "links": [
    { "source": "a", "target": "b" },
    { "source": "b", "target": "c" }
  ]
}

See #9