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

#30 is fixed. 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.

Sorry about the merge commit, I should've created a new branch just for the pull requests.