INT-2866: Add (S)FTP `local-directory-expression`

* Add `local-directory-expression` to (S)FTP Outbound Gateways
* Add `FtpServerRule` to Apache Mina embedded FtpServer
* Add tests for (M)GET and `local-directory-expression`:
FTP tests uses `FtpServerRule`, SFTP tests need testing on real SFTP server

JIRA: https://jira.springsource.org/browse/INT-2866