INT-3172 (S)FTP - Add Recursion Option to LS Cmd

When recursing, the returned filenames are relative to
the top level directory.

INT-3172 (S)FTP - Add Recursion to MGET Command

https://jira.springsource.org/browse/INT-3172

INT-3172 Polishing

- PR Comments
- Add Tests for SFTP (tested with Mock and real SSH)
- Enhance test to ensure subdir is in 3rd file retrieved
- Exclude "special" directories ('.' and '..') from recursion

INT-3172 Make FtpServerRule Safe For Concurrency

Previously the port was static.

- Make the port an instance variable
- Channel the `@ ClassRule` to a `@ Rule`
- Register an appropriate DefaultFtpSessionFactory bean declaration

INT-3172 Polishing; Docs

- Rename FtpServerRule to TestFtpServer
- Remove restriction on filters for MGET
- Add test for filtered MGET
- Reference docs, what's new

INT-3172 More Polish

- Fix type in reference
- Fix error message now that filter is allowed with MGET
- Remove test for filter with MGET (now valid)

INT-3172: Fix tests and typos

INT-3172: this.remoteFileSeparator for recursion