Add random story endpoint

'api/v1/stories/random' selects a random published story. The Category Recorder app relies on this feature.
Can just call the `show` action here, it does some stuff in hal_actions.rb it is easier to just reuse:
``` ruby
  def random
    @story = Story.published.limit(1).order('RAND()').first
    show
  end
```
Alternatively, you could call the same line of code as is in the show action:
``` ruby
  def random
    @story = Story.published.limit(1).order('RAND()').first
    respond_with show_resource, show_options
  end
```