NewClientFromAccessToken turns an accessToken into a new Client.

DefaultUsername controls the default username in the event NoAuth is in effect; if set it will be used, otherwise an API call will be made.

Readonly disables certain actions -- most of which write content back to github -- to ensure testing can be done with a real github client, without affecting too much ON github.

HTTPClient encapsulates the "real world", or http client.

Client is the generic client to github operations.