Update TIdIMAP4Server to support a date/time parameter in IMAP APPEND command

[rlebeau]

(Broken out from #115)

The APPEND command supports an optional date/time parameter to set the mailbox's internal date/time for the appended email, which is separate from the email's Date header. When that parameter is not provided, the server uses the current date/time, regardless of the email's Date header.

TIdIMAP4 has already been updated to support input for that date/time parameter in its AppendMsg...() methods. TIdIMAP4Server needs to be updated to accept that parameter from clients.