Set the User-Agent string to 'Mozilla/4.0' when querying remote servers.

Otherwise, some bad servers return a 403 Forbidden error.
Better description of remote_site_search_url field.
Add field 'remote_site_search_rss_url'.

Now you can set an explicit rss url that we use for querying the
remote site, instead of appending a hardcoded ``search_rss`` to the
site url.  This means we support non-Plone sites now.
Guard against missing remote_site_search_rss_url variable.
Always quote the query and always return a string in these urls.

This is for the extra_result_link and the rss_link.
rss_link could previously return None.
The query could be a string or None.
The query was sometimes quoted and sometimes not.
Now it is always handled the same.
Rebuild i18n. Update Dutch translations.
Merge pull request #1 from zestsoftware/maurits-remote-rss-url

Add remote rss url field