Adjustments for OAuth 1.0a

Further playing with the Twitter API revealed that, now they are enforcing OAuth 1.0a, we need to also send (and sign) the oauth_verifier attribute as well as the oauth_callback. Also, since the special check for 'oauth_token !== false' has been removed lately the entire thing can be removed from ->getParamsToSign() because ->prepareParameters() filters NULL anyways.