Mkdir /usr/local for Homebrew if it doesn't exist

I hit an error when running the laptop script on a new Mountain Lion install.

`chown: /usr/local: No such file or directory`

Homebrew will create and set the mode of the directory, but if we want to chown it for the user prior to installing Homebrew we might need to create it first.

Using `mkdir -p` to only create if it doesn't exist.