Use the item creation date instead of current time for iCal DTSTAMP

Fall back on the current time when the item creation date is missing.

This makes the mail file output deterministic most of the time,
which means it is easier to compare the results.

Leave a FIXME about using the attendee/owner critical change property for
DTSTAMP, since extracting those from the PST data isn't supported yet.

Partially-fixes: https://github.com/pst-format/libpst/issues/9