Connection, Origin: use class instead of struct
Time: use DateTime for StartTime and StopTime
read only exception: 'read-only' instead of 'Read-only'
SessionDescription/TimeCollection: there needs to be at least one time field
bugfixes:
AnnouncedSessionCollection: NotifyCollectionChangedEventArgs add index was wrong
NetworkInterfaceHandler: deletion message only contains origin
SessionDescription: HasTimes checked wrong collection