Add send_meeting_invitations_or_cancellations for UpdateItem

[SebastianBecker2]

As of right now, UpdateItem uses the send_meeting_cancellations enum for its SendMeetingInvitationsOrCancellations attribute. Though it's missing the SendOnlyToChanged and SendToChangedAndSaveCopy.

I'm going to add the send_meeting_invitations_or_cancellations enum, including the respective enum_to_str(...) function for it.

I'm not 100% on the compatibility with Exchange 2007 though. Microsofts docs are a real PITA on the specifics. But at least the EWS Managed Api in Exchange 2007 has an enum for this. So I guess it should be compatible.