support for active hash model adapter

Not sure if this is outside the scope of cancan, but I had a need in my current project for stubbing out resources using active hash (ActiveYaml more specifically) before they could be backed by a proper DB and noticed there was a lack of support. This PR allows you to treat active hash objects like regular model classes in the controller and allows for:

```ruby
# model
class Mock::Stadium < ActiveYaml::Base
  set_filename 'stadiums'
  fields :name, :city, :state
end

# controller
class Api::V1::StadiumsController < Api::V1::BaseController
  load_and_authorize_resource class: Mock::Stadium
end
``` 

Currently it only supports basic scoping and no associations.