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.
Yes, I think the better way is for Nikola to handle the tags automatically (and still relatively) so that intervention is not necessary. Unfortunately, this is outside my skill level. Should one of us make an issue for this?