INT-2981 - Add Rename (mv) to (S)FTP Gateway

* (S)FTP Change Commands, Options to Enums
* INT-2981 Add Support For 'mv' To File/Remote/GW
* INT-2981 (S)FTP Namespace Support and Docs
* INT-2981 Create Remote Dirs if Needed
* INT-2981 Add 'mv' Command to What's New
* INT-2981 Polishing - PR Comments
* Add javadocs to enums.
* Polish schemas to use an enumerated type for available gateway commands
Merge pull request #779 from garyrussell/INT-2981

* garyrussell-INT-2981:
  INT-2981 - Add Rename (mv) to (S)FTP Gateway