Add WORDPRESS_HOST heroku config var

This, together with WORDPRESS_VERSION, allows the buildpack to download a pre-localized Wordpress version.

Example:

```bash
heroku config:set WORDPRESS_HOST=de.wordpress.org
heroku config:set WORDPRESS_VERSION=3.9.1-de_DE
```

This will download Wordpress from http://de.wordpress.org/wordpress-3.9.1-de_DE.tar.gz