Change tmux prefix to C-\

This is `toggle-input-method` in Emacs, which I've never once used, and
doesn't clash for any other program. It's also a lot more ergonomic than
C-z on a MacBook Air.

tmux.conf

@@ -9,10 +9,10 @@ if-shell "test -f ~/.localtmux.conf" 'source ~/.localtmux.conf'
 ## These enables xterm mouse reporting in tmux
 # set -g terminal-overrides 'xterm-256color*:smcup@:rmcup@'
 
-# rebind C-b to C-z
-set -g prefix C-z
+# rebind C-b to C-\ - http://superuser.com/a/271791
 unbind-key C-b
-bind-key C-z send-prefix
+set -g prefix 'C-\'
+bind-key 'C-\' send-prefix
 
 # Shortcut to get to the last window
 bind-key C-a last-window