Prevent false positives when using vip_powered_wpcom() for attribution

Updating the BaseCheck regex to include attribution function in its
pattern.
attribution test should match against http://vip.wordpress.com

adding additional url in attribution link match
Add support for trailing slash for url
Require vip_powered_wpcom

Removes checks for hand-coded links in favor of the requiring use of
the helper function.
escalate level to Blocker and note mentions required function.
Merge pull request #30 from natebot/master

Prevent false positives when using vip_powered_wpcom() for attribution

props @natebot