Serialization Update

• Use strings as ids #13
• Separate id from name in serialization #14

Serialized graphs now look like this:

{
  "nodes": [
    { "id": "95", "propertyName": "fullName" },
    { "id": "96", "propertyName": "firstName" },
    { "id": "97", "propertyName": "lastName" }
  ],
  "links": [
    { "source": "96", "target": "95" },
    { "source": "97", "target": "95" }
  ]
}