Enable the use of parameters in scopes

By passing in the controller it self to the scopes when we evaluate them, we can do stuff like checking controller parameters to check what we should filter with the scope.
Yarr, this is definitely more flexible, but it also requires the user to know that he's got access to `@items`. That should only be a README away, though, so I kind of like this approach a bit more. It's also less hacky, in my opinion. :smiley: