Add IMAP SORT command support.

For more information please visit http://tools.ietf.org/html/rfc5256

PS. I send the pull request on the mronge/libetpan repo for support the IMAP SORT command on the libetpan level in https://github.com/mronge/libetpan/pull/2. But I also send the pull request to the original main in https://github.com/dinhviethoa/libetpan/pull/62 which it's get merged. If you merge the main repo to your libetpan repo. Please discards this pull request.