Add PR.comments, PR.add_comment and PR.update_comment

[samoht]

This is probably not needed as https://api.github.com/repos/samoht/test/issues/32/comments says:

[
  {
    "url": "https://api.github.com/repos/samoht/test/issues/comments/307089478",
    "html_url": "https://github.com/samoht/test/pull/32#issuecomment-307089478",
    "issue_url": "https://api.github.com/repos/samoht/test/issues/32",
    "id": 307089478,
    "user": {
      "login": "samoht",
      "id": 103693,
      "avatar_url": "https://avatars3.githubusercontent.com/u/103693?v=3",
      "gravatar_id": "",
      "url": "https://api.github.com/users/samoht",
      "html_url": "https://github.com/samoht",
      "followers_url": "https://api.github.com/users/samoht/followers",
      "following_url": "https://api.github.com/users/samoht/following{/other_user}",
      "gists_url": "https://api.github.com/users/samoht/gists{/gist_id}",
      "starred_url": "https://api.github.com/users/samoht/starred{/owner}{/repo}",
      "subscriptions_url": "https://api.github.com/users/samoht/subscriptions",
      "organizations_url": "https://api.github.com/users/samoht/orgs",
      "repos_url": "https://api.github.com/users/samoht/repos",
      "events_url": "https://api.github.com/users/samoht/events{/privacy}",
      "received_events_url": "https://api.github.com/users/samoht/received_events",
      "type": "User",
      "site_admin": false
    },
    "created_at": "2017-06-08T12:33:55Z",
    "updated_at": "2017-06-08T12:33:55Z",
    "body": "test"
  }
]

but https://api.github.com/repos/samoht/test/pulls/32/comments says:

[

]