object.id should always work

[tobixen]

object.id is sometimes set and sometimes not.

Move it to object._id and make some getter method that will dig into the data to find the uid if self._id is not set.

[tobixen]

See also #104.

[tobixen]

This is actually a duplicate of #96

[tobixen]

To clarify, event.id should always work and return the uuid for CalendarObjectResource objects sans FreeBusy. I'm still not sure if it makes any sense to have an id field in other DAVObjects.

[tobixen]

I've seen that event.id cannot always be trusted. We need some unit tests and functional tests on this, covering all kind of edge-cases.