Check to see if Brew is installed

[Brooks Swinnerton]

Rather than immediately installing Brew, it'd be nice if there was a check for the executable before installing it:

  Installing Homebrew for you.
It appears Homebrew is already installed. If your intent is to reinstall you
should do the following before running this installer again:
    ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/uninstall)"