[Feature] Rails 2 support

There are still a lot of large production apps running Rails. So are we. I would like to start using Pundit with Rails 2 while we're still in the process of migrating our app to Rails 3.

Since Pundit is such a small and clean gem it proved to be quite easy to backport to Rails 2.

I would be very grateful if you could merge this pull request. I've seen