Current user in rules

Addresses https://github.com/tute/merit/issues/98 feature request:

> Is there anyway I can pass the action user to the block ?
>
> ```ruby
> score 1, log: 'Reading', to: :reader, on: 'news#show' do |news, action_user|
>   User.points_already_rewarded(action_user.id, 'show', 'news', news.id) < 2
> end
> ```
> I need to pass in the user as we will run the unprocessed entries in the background as a cron and won't have access to the session state. Also, I have no way to get to the user from the news object.