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`
[Our guides state](https://github.com/thoughtbot/guides/tree/master/style#unit-tests):

> Use a single, top-level describe `ClassName` block.

This seems to be breaking that guideline. Is there a reason?