Improve date/time widgets
The current date/time widget is not very lazy on error encoding. It also enforce too much the order in which a date must be filled.
Those issues could be improved by using the parser [1] of python-dateutil.
Also we can just use the local of the client user to format the date (like sao) and thus having a format more complex but still allow entering simple format (supported by dateutil).
[1] http://labix.org/python-dateutil#head-a23e8ae0a661d77b89dfb3476f85b26f0b30349c