add support for syncing with other: and shared: via NAMESPACE

finish implementing --quiet

--fast downloads the last message again if no new messages have arrived

isync gets confused when new mail is delivered while in the middle of an
IMAP session.  need to handled those asynchronous notifications properly.

in order to be in total compliance with the maildir(5) spec, we need to
add code to clean the tmp directory on the 24-hour timer.