Changes in login and token obtaining procedures,
token is now stored and can be reused when new token can be downloaded
More changes in token getting procedure, now you can specify whether
you want new token or try to reuse stored one
Small style and code fixes
Further work on `Notificatin()` model; `str()` and `repr()` support,
`who()` and `when()` methods implemented
`Notification().who()` now returns list of guids af all users mentioned
in notification (not only first one).