Fixes #2268 Implement a default Referrer spam blacklist to block semalt.com spammer. (can be extended to other spammers on request!)

; All Visits with a Referrer URL host set to one of these will be excluded.
; If you find new spam entries in Referrers>Websites, please report them here: http://dev.piwik.org/trac/ticket/2268
; List of known Referrer Spammers, ie. bot visits that set a fake Referrer field:
referrer_urls_spam = "semalt.com"