def status(mailbox, attr)
      synchronize do
        send_command("STATUS", mailbox, attr)
        return @responses.delete("STATUS")[-1].attr
      end
    end
Sends a STATUS command, and returns the status of the indicated
+mailbox+. +attr+ is a list of one or more attributes that
we are request the status of.  Supported attributes include:

MESSAGES:: the number of messages in the mailbox.
RECENT:: the number of recent messages in the mailbox.
UNSEEN:: the number of unseen messages in the mailbox.

The return value is a hash of attributes. For example:

p imap.status("inbox", ["MESSAGES", "RECENT"])
#=> {"RECENT"=>0, "MESSAGES"=>44}

A Net::IMAP::NoResponseError is raised if status values
for +mailbox+ cannot be returned, for instance because it
does not exist.