Make response headers available to user-code

Originally, the Client.request() method did not pass the response
headers back to the user, to avoid unnecessary distraction. But,
obviously, the headers will often be needed for further processing.
Therefore, we make them available as the Github.getheaders() method.

- Client.request()
    stash away the headers of the last response in self.headers

- Github.getheaders()
    retrieve the headers from the client
Update TODO file