Make pull request review comments a subclass of issue comments.

Based on some discussion in #99 I believe its safe to use html_url for PullRequestReview comments.

I did some basic checking on URLs like this:

https://api.github.com/repos/octokit/octokit.objc/pulls/110/comments 

and ```html_url``` was available. Also ```pull_request_url``` is available. Not sure if using that would be better thank the _links stuff.