Add support for SORT command defined in IMAP RFC 5256
http://tools.ietf.org/html/rfc5256
Refactor IMAP SORT command support to use the extension API.
Added free sort result list function.
Fixed: Cyclic include.
Merge pull request #62 from pitiphong-p/imap_sort

Add support for SORT command defined in IMAP RFC 5256