Fix bug #30. Show comments now works with Github API v3.

Github API v3 now separates issue comments, pull request commits, and comments on those commits. Currently the code is able to show issue comments and commits in chronological order.

However I haven't found a reliable way to show comments on the commits inside a pull request, as those comments are actually in the forked repo where the pull request was coming from.

Also, more tests are needed to cover the `discussion` method. I'm working on that.