Switch to authenticated access to the Github API. Otherwise requests are...

... limited to 60 an hour.

GH limits unauthenticated requests to 60 per hour. There are already over 40 projects, soon it will be impossible to refresh all their data without hitting the limit, or slowing execution down to > 1 hour.

When an OAuth token is included one can do 5000 requests an hour. This PR contains the necessary changes, and updates the README to explain how to get the token.