def for_each_time_range(t)
      RedisAnalytics.redis_key_timestamps.map{|x, y| t.strftime(x)}.each do |ts|
        yield(ts)
      end
    end
def for_each_time_range(t)
      RedisAnalytics.redis_key_timestamps.map{|x, y| t.strftime(x)}.each do |ts|
        yield(ts)
      end
    end
This class represents one unique visit
 User may have never visited the site
 User may have visited before but his visit is expired
 Everything counted here is unique for a visit
 helpers