Add hooks for the login and register phases

I needed to add an option for overriding the /auth/identity and /auth/identity/register routes so I can call my own action on those routes. I used the same pattern already in place for the on_failed_registration option.

```

  provider :identity,
    :fields => [:nickname],
    :locate_conditions => lambda { |req| { model.auth_key => req['nickname']} },
    :on_login => SessionController.action(:login),
    :on_register => SessionController.action(:register),
    :on_failed_registration => SessionController.action(:failure)

```