Handling of Unpermitted Keys

Cherry-picked and fixed up @thomasfedb's work on unpermitted keys to work with Rails 2.x

Only difference between the Rails 3+ version is we need to call `ActionController::Parameters.action_on_unpermitted_parameters` directly via an initializer. That change is noted in the README. 

Tests are passing and it's working in my Rails 2.x app. Let me know if there is anything you'd like me to tweak or cleanup :smile: