Add tmux to common system packages

When I do have to login to a machine, it's nice to have a terminal-
multiplexing capability and let me handle flakey VPN connections in
a graceful manner, so that any child processes aren't accidentally
killed.

Not sure if we should do it like this, or like we did for GOV.UK.

Thoughts?
Add tmux module

This will give us screen keybindings, as per Sam's suggestion.
Use tmux class from module

Remove system package, since we want screen keybindings.
Merge pull request #13 from alphagov/tmux

Add tmux to common system packages