IMAPClient should provide easy access to unsolicited responses

[mjs]

Originally reported by: Menno Smits (Bitbucket: mjs0)

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_imap.untagged_responses should be accessible in sane way to allow clients to know about unsolicited messages from the server.

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• Bitbucket: https://bitbucket.org/mjs0/imapclient/issue/155

[bengotow]

This would be really nice. We're seeing an issue at Nylas right now where many IMAP connections are being closed by the remote server, and I think / hope that we may be receiving an unsolicited ALERT from the server prior to the socket being closed, but it's not easily accessible.

[mjs]

OK, attaching it to 1.1.0 to make sure this isn't forgotten.