install tmux debian package in common role

TMUX is a terminal multiplexer similar to GNU screen but with many improvements. See their FAQ for differences/improvements:

http://sourceforge.net/p/tmux/tmux-code/ci/master/tree/FAQ

also:

http://www.techrepublic.com/blog/linux-and-open-source/is-tmux-the-gnu-screen-killer/
I agree that if we're putting screen on every machine, we can also put tmux.