Clean up code

• FolioReader.clear();

  Nullifies readPosition and listeners.
  This method ideally should be used in onDestroy() of Activity or Fragment.
  Use this method if you want to use FolioReader singleton instance again in the application,
  else use FolioReader.stop() which destruct the FolioReader singleton instance.
  

• FolioReader.stop();

  Destructs the FolioReader singleton instance.
  Use this method only if you are sure that you won't need to use
  FolioReader singleton instance again in application, else use FolioReader.clear().