Class POP3

Known Subclasses:

• POP3_SSL

This class supports both the minimal and optional command sets.
Arguments can be strings or integers (where appropriate)
(e.g.: retr(1) and retr('1') both work equally well.

Minimal Command Set:
        USER name               user(name)
        PASS string             pass_(string)
        STAT                    stat()
        LIST [msg]              list(msg = None)
        RETR msg                retr(msg)
        DELE msg                dele(msg)
        NOOP                    noop()
        RSET                    rset()
        QUIT                    quit()

Optional Commands (some servers support these):
        RPOP name               rpop(name)
        APOP name digest        apop(name, digest)
        TOP msg n               top(msg, n)
        UIDL [msg]              uidl(msg = None)

Raises one exception: 'error_proto'.

Instantiate with:
        POP3(hostname, port=110)

NB:     the POP protocol locks the mailbox from user
        authorization until QUIT, so be sure to get in, suck
        the messages, and quit, each time you access the
        mailbox.

        POP is a line-based protocol, which means large mail
        messages consume lots of python cycles reading them
        line-by-line.

        If it's available on your mail server, use IMAP4
        instead, it doesn't suffer from the two problems
        above.

Authorisation

- only possible if server has supplied a timestamp in initial greeting.

Args:
        user    - mailbox user;
        secret  - secret shared between client and server.

NB: mailbox is locked by server from here to 'quit()'