Add support for scheduling meetings

[bkircher]

In particular:

• implement item property <ResponseObjects/> (getter, seems to be read-only)
• create meetings and inviting attendees (via <CreateItem/> and <UpdateItem/>)
• modifying attendee collections (via <SetItemField/> works already. TODO here: <DeleteItemField/> should remove all attendees, <AppendToItemField> should add one)
• forwarding meetings and meeting requests
• support for meeting messages
• creating response objects, <AcceptItem/>, <DeclineItem/>, and <TentativelyAcceptItem/> elements
• cancelling meetings
• support for recurring meetings