Add support for cross references

[bkeepers]

$ curl --header "Accept: application/vnd.github.mockingbird-preview+json" \
  https://api.github.com/repos/bkeepers/dotenv/issues/75/timeline
....
  },
  {
    "source": {
      "id": 18472342,
      "actor": {
        "login": "metavida",
        "id": 1933,
        "avatar_url": "https://avatars.githubusercontent.com/u/1933?v=3",
        "gravatar_id": "",
        "url": "https://spider-skull-island.github.com/api/v3/users/metavida",
        "html_url": "https://github.com/metavida",
        "followers_url": "https://spider-skull-island.github.com/api/v3/users/metavida/followers",
        "following_url": "https://spider-skull-island.github.com/api/v3/users/metavida/following{/other_user}",
        "gists_url": "https://spider-skull-island.github.com/api/v3/users/metavida/gists{/gist_id}",
        "starred_url": "https://spider-skull-island.github.com/api/v3/users/metavida/starred{/owner}{/repo}",
        "subscriptions_url": "https://spider-skull-island.github.com/api/v3/users/metavida/subscriptions",
        "organizations_url": "https://spider-skull-island.github.com/api/v3/users/metavida/orgs",
        "repos_url": "https://spider-skull-island.github.com/api/v3/users/metavida/repos",
        "events_url": "https://spider-skull-island.github.com/api/v3/users/metavida/events{/privacy}",
        "received_events_url": "https://spider-skull-island.github.com/api/v3/users/metavida/received_events",
        "type": "User",
        "site_admin": false
      },
      "url": "https://spider-skull-island.github.com/api/v3/repos/bkeepers/dotenv/issues/53"
    },
    "created_at": "2013-11-24T16:47:55Z",
    "updated_at": "2013-11-24T16:47:55Z",
    "event": "cross-referenced"
  },
....