Implement GitHub.API.Issue.comments(GithubIssue.t)

[begedin]

Problem

In order to perform the comments step of syncing a github repository to codecorps, we need to implement GitHub.API.Issue.comments(GithubIssue.t). It accepts a GithubIssue and returns the list of comments for that issue.

This depends on the implementation in #1107

[joshsmith]

I'm closing this because I think it's not needed. We'll fetch the comments all at once with #1111.