class method .resource

There are some cases when it is nesessary to carry resourse to class method in authorizer. For example we have:

```
class ApplicationAuthorizer < Authority::Authorizer
  def self.readable_by?(user)
    user.has_permission?(:read, resource)
  end
end
```

When we call `Comment.readable_by?(user)`, we call `ApplicationAuthorizer.readable_by?(user)`, and in `readable_by?` method we have no connection with model it is called on.

The easiest way is to send it as param to method call, but there can be some compatibility issues with earlier versions of gem.

 Here i tryed to wrap authorizer class with another class where i define a class instance variable with resource class through flat scope. I'm not quite sure if this approach is the best way to carry resource class to authorizer. What do you think?