Rename methods for adding comments

* I have a feature branch that implements issue commenting. This commit
  renames methods in `PullRequest` and `GithubApi` in anticipation of the
  changes on my branch.
* Rename `PullRequest#add_comment` to `PullRequest#comment_on_violation`
* Rename `GithubApi#add_comment` to `GithubApi#add_pull_request_comment`