Set a DTSTAMP for new VEVENT/VTODO components

[WhyNotHugo]

VTODO (amongst others) components MUST a DTSTAMP entry, and it should contain the timestamp of the components creation.

I'd like to propose that creating new components of these types have this value automatically added, with datetime.now as a default.

This will basically make these components standards-compliant (and the value is an expected one too).

[geier]

icalendar's current approach is to let the user handle everything themselves. Convenience functions, like the one suggested here, could go hand in hand with implementing a validation checker (as mentioned in #28). This could be a point for the 4.0 roadmap (#186) if anyone is interested in working on it.

[untitaker]

My position is that icalendar really shouldn't let the user produce invalid data by default. A validation checker is opt-in.