Added 'client' authentication to allow increasing rate limit

The rate limit for unauthenticated calls is 60 calls per hour. This may be increased to 5000 per hour by supplying the client ID & client secret as GET parameters to each request.

> Unauthenticated rate limited requests
If you need to make unauthenticated calls but need to use a higher rate limit associated with your OAuth application, you can send over your client ID and secret in the query string.

From: http://developer.github.com/v3/#unauthenticated-rate-limited-requests

I've added the `client` authentication type that requires both the `clientID` and `clientSecret` parameters.