Create send_directory backend command

I'm creating [remote_directory resource](https://docs.chef.io/resource_remote_directory.html) on itamae [★](https://github.com/k0kubun/itamae/commit/87dfc32baa34a84d23fc4e9b9277e0f29076da84). But sending directory via Net::SCP requires `:recursive` option [★](https://github.com/net-ssh/net-scp/blob/7677524617cc733b5ee0b15f090ed81a361e0600/lib/net/scp.rb#L335), so I couldn't send directory with `send_file`.

Thus I made send_directory backend API to send a directory.