osmupdate: retreave timestamp form .pbf file

```dowload.py``` tries to retrieve the latest timestamp from ```country.pbf``` using ```osmconvert country.pbf --out-timestamp```. 
If the ```country.pbf``` file does not have a timestamp and, ```osmconvert``` returns ```(invalid timestamp)```, ```download.py``` could recover the last-timestamp using ```osmconvert country.pbf --out-statistics```, or is should throw an exception.   