Added support for repository comments

[georgebrock]

In doing so I broke backward compatibility (by renaming the existing comments method on the Repo class to issue_comments) so you may not want to merge this in as-is.

[naktinis]

You could rename it to repo_comments for example.

[georgebrock]

The method is on a Repo class: having Repo.comments return issue comments and Repo.repo_comments return repo comments seems very confusing to me.