tmux: easier command prefix

Since CTRL-b is a rather unwieldy shortcut I remapped it to CTRL-a in ~/tmux.conf.

unbind C-b
set -g prefix C-a
bind C-a send-prefix

And since I have no need for CapsLock I remapped that to an extra control key in my OSX keyboard settings ("Keyboard" pref pane, "Modifier Keys..."), so the 2 keys needed for the tmux prefix are right next to each other now.