Manage Unrealized Gains/Losses.

    In multi-currencies environments, we need a way to control the risk related
    to currencies (in case some are higthly fluctuating) and, in some countries,
    some laws also require to create journal entries to record the provisionning
    of a probable future expense related to currencies. Hence, people need to
    create a journal entry at the beginning of a period, to make visible the
    probable expense in reports (and revert it at the end of the period, to
    recon the real gain/loss.
    