Add key binding for window joins

This comes from [1] except I don't want the other command (it also unbinds
the session change key).

Also found that `!` is already bound to `break-pane` and does what I
want. [2]

[1]: http://unix.stackexchange.com/questions/14300/moving-tmux-window-to-pane

[2]: http://www.openbsd.org/cgi-bin/man.cgi?query=tmux&sektion=1