Comment on existing issue on Github.
        For JSON data returned by Github refer:
        https://developer.github.com/v3/issues/comments/#create-a-comment

        :param issue: object of exisiting issue
        :param body: body of the comment
        :returns: dict of JSON data returned by Github of the new comment
        :rtype: `dict`

def comment(self, issue, body):
        """Comment on existing issue on Github.
        For JSON data returned by Github refer:
        https://developer.github.com/v3/issues/comments/#create-a-comment

        :param issue: object of exisiting issue
        :param body: body of the comment
        :returns: dict of JSON data returned by Github of the new comment
        :rtype: `dict`

        """
        url = issue.comments_url
        data = {'body': body}

        response = self.session.post(url, json.dumps(data))
        assert response.status_code == 201

        return json.loads(response.content)