Fix #314: add ssh+tmux wrapper scripts

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.
Merge pull request #315 from ufo-kit/remote-tmux-connection

Fix #314: add ssh+tmux wrapper scripts