WIP: Add CSRF token for write operations

#98
Is there a reason not to use the `restful_json_output` delivery callback and do:

```php
function restful_csrf_session_token() {
  drupal_add_http_header('Content-Type', 'text/plain');
  return drupal_get_token('rest');
}
```

It seems more consistent with the rest of the module's output. It will add some wrapping `"` though.