Add initial pass for creating review comments

See #284 for more context.

TODO:

- Add tests
  - [x] Integration
  - [x] Unit

- Allow user to [reply](https://developer.github.com/v3/pulls/comments/#create-a-comment) to a particular review comment.
  - [x] Add method to ReviewComment if practical
  - Add tests
    - [x] Integration
    - [x] Unit