Adding Identity, Claim and proof

There are many cases in social systems where third party identities are associated with the main identity (e.g. linking facebook and twitter accounts to a user account). Unfortunately, there are no existing common models or standards to draw on in terms of how these are represented after the fact. Introducing Identity, Claim and proof provide a way.

```json
{
  "@type": "Claim",
  "actor": "acct:sally@example.org",
  "object": {
    "@type": "Identity", 
    "@id": "acct:sally@example.net"
  },
  "proof": {
    "@type": "...",
    "@value": "..."
  }
}
```