Let's not hard code the arbitrary _access_token_ param name and centralize the value in the plugin definition.

```php
$plugin = array(
  'label' => t('Token based authentication'),
  'description' => t('Authenticate requests based on the token sent in the request.'),
  'name' => 'token',
  'options' => array('param_name' => 'access_token'),
  'class' => 'RestfulAuthenticationToken',
);

```