Add ssh protocol, which would call into external ssh/scp

Ruby's SSH/SFTP uses OpenSSH, which doesn't work nicely with some
commercial SSH libraries. New protocol code uses external ssh/scp
tools.

To simplify implementation, it's currently expected the remote to
be a Linux machine to execute commands through ssh:

    - find
    - mkdir
    - stat
Refactor ssh protocol code

Include fixes for clonezone comments:

    - use Fig::ExternalProgram instead of ``
    - add "-n" option to ssh command line
    - use better error messages
    - fix new class and function comments
Merge pull request #72 from klimkin/sys-ssh

Add ssh protocol via external ssh/scp commands.