robots: add newsblur fetchers to the list of stealth robots.

This not only ensures that robots limitations are applied properly to
newsblur fetchers, but would also allow them to request the feeds, as
their user agent is faked and would be stopped by the 60 ruleset.

User-Agent: NewsBlur Feed Fetcher - 5 subscribers - http://www.newsblur.com (Mozilla/5.0 (Macintosh; Intel Mac OS X 10_7_3) AppleWebKit/536.2.3 (KHTML, like Gecko) Version/5.2)
User-Agent: NewsBlur Page Fetcher (5 subscribers) - http://www.newsblur.com (Mozilla/5.0 (Macintosh; Intel Mac OS X 10_7_1) AppleWebKit/534.48.3 (KHTML, like Gecko) Version/5.1 Safari/534.48.3)
User-Agent: NewsBlur Favicon Fetcher - http://www.newsblur.com