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

INT-2866: Documentation

INT-2866 Polishing

- Remove leading /
- Change \ to / in invalid test
- Clean up after sftp

Tested with real SSH.

INT-2866 Polishing - Add Mock SFTP Test

Run with -Dspring-profiles-active=realSSH to run with a real SSH server.

Assumes ftptest/ftptest account on localhost with the following directory tree in the user's root...

  $ tree sftpSource/
  sftpSource/
  ├── sftpSource1.txt
  ├── sftpSource2.txt
  └── subSftpSource
      └── subSftpSource1.txt

INT-2866: Polishing

INT-2866: change `remotePath` to `remoteDirectory`

Doc Polishing.