Made _logic a param
The _logic parameter is added in base.py to make this as small and
non-intrustive change.
Patch by: Sverre Rabbelier
# Directions for web crawlers.
# See http://www.robotstxt.org/wc/norobots.html.
User-agent: HTTrack
User-agent: puf
User-agent: MSIECrawler
User-agent: Nutch
Disallow: /