Correct folder argument sequence in README
Register command and middleware

Also, output a log when Vagrant::Mirror::Middleware::Sync runs and there
are no folders configured.
Net::SFTP creates directories as part of recursive transfer

Do not manually create new directory in Vagrant::Sync::All as this
will cause recursive upload to fail
Stub Vagrant::Mirror::Connection::SFTP.close

Should close the SFTP and SSH connections when implemented
Net::SFTP Dir.entries does not return array of names

dir.entries returns an array of Name objects, with attributes and other
data, unlike Dir.entries. Convert to array of filenames.