Add RequestedReviewers to PullRequestPayload

[kuiro5]

This PR adds support for the optional requested_reviewers attribute for the review_requested action as part of thepull_request event. This is useful for consuming which Github users have been added to a pull request.

request_reviewers:

{
  "requested_reviewers": [
      {
        "login": "foo",
        "id": 123,
        "avatar_url": "https://avatars2.githubusercontent.com/",
        "gravatar_id": "",
        "url": "https://api.github.com/users/foo",
        "html_url": "https://github.com/foo",
        "followers_url": "https://api.github.com/users/foo/followers",
        "following_url": "https://api.github.com/users/foo/following{/other_user}",
        "gists_url": "https://api.github.com/users/foo/gists{/gist_id}",
        "starred_url": "https://api.github.com/users/foo/starred{/owner}{/repo}",
        "subscriptions_url": "https://api.github.com/users/foo/subscriptions",
        "organizations_url": "https://api.github.com/users/foo/orgs",
        "repos_url": "https://api.github.com/users/foo/repos",
        "events_url": "https://api.github.com/users/foo/events{/privacy}",
        "received_events_url": "https://api.github.com/users/foo/received_events",
        "type": "User",
        "site_admin": false
      }
    ]
}

Thanks, let me know if you need anything else!

[deankarn]

@kuiro5 sorry for the delay will try to get this merged today