RequestHandler: Allow setting of the `baseUrl` to use. 

This allows us to use it for OAuth endpoints (e.g. oauth/request_token). These endpoints are on "www.tumblr.com" and not on the "api" subdomain. Example usage:

    $client->getRequestHandler()->setBaseUrl('http://www.tumblr.com/');
    $req = $client->getRequestHandler()->request('POST', 'oauth/request_token', [
      'oauth_callback' => '...',
    ]);

The default `baseUrl` is "http://api.tumblr.com/".