Fix #314: add ssh+tmux wrapper scripts

I thought about using the IPython kernel stuff but it seemed more work for very little gain. In the end we could have just attached to a running Concert session. This PR does the same but uses the right tool for the job, i.e. sharing sessions through terminal multiplexing. What neither this nor IPython provide is remote control of a session but this sounds fishy to me no matter what.

From the commit message: The `concert-server` script starts a new tmux session concert-PID and starts a concert session with arguments passed to the start command. The `concert-connect` script connects via ssh to a remote machine where such a tmux session must be running and attaches itself to it.