refactored to enable creating new gist with all options / moved Complete method to GitHubApiBase to allow global handling of request completion (invalidate Authorization on Not Authorized, RateLimit tracking) / implemented RateLimit tracking and prevented new requests if expired / moved POST and DELETE methods to GitHubApiBase to enable global request post-processing / created Anonymous Authorization to support anonymous RateLimit tracking / removed unused Application property from GitHubApiContext / moved Authorize(Authorization) and ClearAuthorization to GitHubApiContext / updated TestAccount mechanism to invalidate expired/removed authorizations / updated tests