Fix problem with local search when site_url is not at the root

When the site url is not at the root, the localsearch plugin __init__.py needed to be modified slightly to include the folder when using the post permalink. In addition, I added some corresponding comments to the conf.py.sample.
I did think of that way first, but it makes the content.json file contain absolute paths to the SITE_URL. This way the content.json file may be used on any site that has the same folder path structure.