Inheritance: extends Illuminate\Database\Eloquent\Model
 /**
  * Link the give user to the given provider.
  *
  * @param integer $user_id
  * @param string  $provider
  * @param integer $provider_id
  * @param array   $access_token
  * 
  * @return Provider
  */
 protected function linkProvider($user_id, $provider, $provider_id, $access_token)
 {
     $user_provider = new Provider();
     $user_provider->user_id = $user_id;
     $user_provider->provider = strtolower($provider);
     $user_provider->provider_id = $provider_id;
     $user_provider->access_token = $access_token;
     $user_provider->save();
     return $user_provider;
 }