Feature: support icalendar objects about caldav

[tobixen]

There exists one or two pull requests already, but none of them are backward-compatible as far as I know.

I've taken one step in the right direction, I've renamed "instance" to "vobject_instance", and made "instance" point to "vobject_instance" for legacy reasons. I've also delayed the object instantiation (ref #43), so that the vobject logic will only be called on when it's needed.

My next ideas:

• fix an "icalendar_instance" that returns an icalendar instance. As edits to the instance should be honored, only one instantiated object can be attached to the caldav object - if obj._vobject_instance is not None, then first set obj._data=obj._vobject_instance.serialize() and then set obj._vobject_instance to None before setting obj._icalendar_instance, and vice versa. (This won't be very optimal if both object types are frequently accessed, but that shouldn't really happen)
• Do some more research on it - is icalendar really better? The main argument for moving to icalendar was that vobject didn't support python3 and wasn't maintained. I also tried looking a bit into vobject myself to fix some bugs and such - but I found a rabbit hole - I didn't want to go deeper there. At least the maintainer situation has improved as far as I can understand ... maybe things aren't that bad anymore?
• If we decide to ditch vobject, then all code accessing the instance should be rewritten to support icalendar, the import of vobject should be done if/when someone calls on the vobject_instance, and vobject should be marked an optional dependency. (test code still needs to exercise this code path)