: list(refname, mailbox)
      Sends a LIST command, and returns a subset of names from
      the complete set of all names available to the client.
      The return value is an array of imap.rb"/Net::IMAP::MailboxList.

      ex).
        imap.create("foo/bar")
        imap.create("foo/baz")
        p imap.list("", "foo/%")
        #=> [#<Net::IMAP::MailboxList attr=[:Noselect], delim="/", name="foo/">, #<Net::IMAP::MailboxList attr=[:Noinferiors, :Marked], delim="/", name="foo/bar">, #<Net::IMAP::MailboxList attr=[:Noinferiors], delim="/", name="foo/baz">]

