Improved library.

- I implemented a better datacontainer class(es) with
  the very base commands add, update and delete.
- The Note class was simplified.
- With NoteJSONEncoder the Note class can be serialised
  to JSON.

Signed-off-by: Stefan Tatschner <stefan@sevenbyte.org>
Implemented new_note, show_note and delete_note.

The data is now stored in an instance dict with int keys.
It will be easier to access data because we do not have
to search the corresponding object in a list. We can kust
access it by the key.

Signed-off-by: Stefan Tatschner <stefan@sevenbyte.org>