Refactor

* added vendor to .gitignore for anyone that vendors gems
* added explanations to the README for the default webtrends tags
* analytics_visitor_id now returns a string by default
* removed the .to_s from the param passed to //.match(x) since we did not
  need it any more
* made Webtrends::Rails::Controller methods private
* changed controller.analytic_tags to analytics_tags in the track method