Add a variation of the ssh client that uses scp

The `scp` client is based on the ssh client & does everything the same way except *writing* files (and only writing files at the moment). It stores any file it's asked to write in a tmpfile, spawns an scp child process, then unlinks the tmpfile when done. 

You can specify the use-scp variation of the ssh client by substituting `scp` in any working `ssh` configuration.