:if option that accepts lambdas added to skip middleware

`:if` option added to skip google analytics if returns false. 

A `lambda` might be used, so we can do something like 

    Rack::GoogleAnalytics, 
        :tracker => lambda { |env| return env[:ga_tracking_code] },
        :if => lambda { |env| return !env[:ga_tracking_code].nil? }

We can check for any of the variables of in `env`.

It would be possible to disabled GA for `controllers` or `controllers actions` using `before_filter`