Add /usr/local/sbin to PATH in OSX

I recently installed RabbitMQ via Homebrew, and ended up having to add `/usr/local/sbin` to my PATH. After discussing with the Homebrew devs on IRC, it came up that `/usr/local/bin` is only a cultural convention, and that Homebrew may install a package anywhere in `/usr/local`.

I've not found anything that installs outside `/usr/local/bin` and `/usr/local/sbin`.