Fallback to use the "LIST" defined in "IMAP4rev1" RFC if server doesn't support "XLIST" command (eg. Yahoo)

Some IMAP server eg. Yahoo doesn't support the "XLIST" command. This pull request fallback to the "LIST" command defined in IMAP4rev1 (RFC 3501 http://tools.ietf.org/html/rfc3501).
Merge branch 'refs/heads/fallback_to_list_command' into private_branch