Add functionality to sample backend

* set_properties
* save (message)
* delete_message
* create_attachments
* get_attachment_ids
In python world, better the old version:

    for item in list:
      if item_found:
          return whatever
    return None

In ruby this would had been nicer :dancers: :

    self.basedict[:subfolders].select{|i| i[:fid] == folderId }