/**
  * Retrieves the one true instance of Segment_Analytics
  * Also sets up constants and includes deprecated files.
  *
  * @since  1.0.0
  * @return object Singleton instance of Segment_Analytics
  */
 public static function get_instance()
 {
     if (!isset(self::$instance)) {
         self::$instance = new Segment_Analytics();
         self::$instance->setup_constants();
         if (!has_action('plugins_loaded', array(self::$instance, 'include_deprecated_files'))) {
             add_action('plugins_loaded', array(self::$instance, 'include_deprecated_files'), 20);
         }
     }
     return self::$instance;
 }