Add syntax and setup support for tmux.

I've had `tmux.vim` lying around for a while.  I don't edit `tmux.conf` much, but it's nice to have this when I do.  Feel free to reject it if you thinks it's too specialized though.

It's not really a plugin, so I didn't know where to document it.  I used the following to obtain the latest copy of `tmux.vim`:

    wget http://sourceforge.net/p/tmux/tmux-code/ci/master/tree/examples/tmux.vim\?format\=raw -O syntax/tmux.vim