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.
INT-3147: (3167,3173,1941) Improve MetadataStore

Previously, `MetadataStore` couldn't be configured for Twitter Adapters
- only global one.
The `metadataKey` was generated automatically with 'difficult' value

* Register all `MessageSource` for `SourcePollingChannelAdapter`
as beans with id based on adapter id and prefix '.source' (INT-3147)
* Polishing parser to get rid of explicit `MessageSource` beans. (INT-3147)
* Make Feed and Twitter adapters `id` attribute as required -
now it presents a `metadataKey` for `MetadataStore` (INT-3147)
* Add to Twitter adapters a reference attribute for `MetadataStore` (INT-3173)
* Add Twitter adapters `poll-skip-period` attribute (INT-3167)
* Add and implement `MetadataStore#remove` (INT-1941)
* Make `MetadataStore` as `@ManagedResource` (INT-1941)
* Polishing tests

JIRAs:
https://jira.springsource.org/browse/INT-3147
https://jira.springsource.org/browse/INT-3167
https://jira.springsource.org/browse/INT-3173
https://jira.springsource.org/browse/INT-1941