Disallow indexing when environment isn't production

Add a mu-plugin that will disallow robots/indexing if the environment isn't production. 

The plugin will just set the [`blog_public` option](http://codex.wordpress.org/Option_Reference#Privacy) to false, which takes care of adding the robots meta (noindex,nofollow) to `<head>`, along with adding the proper `Disallow` to `robots.txt`