Make it easier to use imapdedup.py as a Python module

Thanks for writing this!

These changes make it easy to start the deduplification in an automated way (e.g., from a cron job) via a short file that can be as simple as:

    import imapdedup

    class options:
        server = 'imap.example.com'
        port = None
        ssl = True
        user = 'me'
        password = 'Pa$$w0rd'
        verbose = False
        dry_run = False
        use_checksum = False
        use_id_in_checksum = False
        just_list = False

    mboxes = [
        'INBOX',
        'Some other mailbox',
        ]

    imapdedup.process(options, mboxes)

This, in turn, is nice because it doesn't require a password to be passed to the program via a command-line argument, where it could be seen by other users of the system.  (This short startup file could be made read-only.)