Support unauthenticated rate limited requests

This PR adds support for unauthenticated rate limited requests (http://developer.github.com/v3/#unauthenticated-rate-limited-requests), which are subject to the higher rate limit of your OAuth application but are not authenticated with any user's Github credentials.

If you are interested in merging this, let me know what kind of tests would you like to see and I will add them to this PR.

(Credit: I copied cloneRequest from code.google.com/p/goauth2/oauth.)