Limit lookups to specific classes via :only

Seems straightforward.  The ticket mentioned limiting via `klass < mod` but I would think `klass <= mod` an implemented accordingly...
I think I like that approach more actually.

```ruby
class << self
  def find(gid, options = {})
    if global_id = parse(gid)
      global_id.find options
    end
  end
end

def find(options = {})
  if find_allowed_for?(klass, options[:only])
    model_class.find model_id
  end
end
```