Bump version to 0.1.2
Enable sftp subsystem in cucumber sshd config
Add net-sftp gem dependency
Import Net::SSH::Test::Channel code from net-sftp:

  This class is included in net-ssh, but net-sftp adds some new methods
that we need for our tests. This code resides in net-sftp test helper,
and can't be required dynamicaly, this change adds a copy of the code we
need, hardcoded in NetSSHStoryHelpers.
Implement NetSSHStoryHelpers#sftp_story
Add Remote::FS class
Implement Remote::FS#sftp
Implement Remote#fs
Implement Remote::FS#has_file?
Implement `has_file' condition keyword
Bump version to 0.1.3