#57 Purge should create unbalanced record

[Matija Nalis]

When Purging old data, new (unbalanced) record should be
created in all accounts affected, dated with date
purged, and with ammount equal to all data purged and
with name like "Purged old data".

that way, account balance can be rebuilt if needed (to
undo "reset I/E", or if things go wrong anywhere)