[Feature] Notifications api for activities

The api is used like:

    self.notify('Bob sent you DOGE 5',
                'Come back to your doge coin wallet to spend your DOGE!')

The api is found in activity.notify

This requires https://github.com/tchx84/sugar/pull/2 (which is built on https://github.com/sugarlabs/sugar/pull/267 FYI).