Add sub navigation menu for nested resources

*Warning, this is still work in progress.*

![Nested resource sub navigation](https://f.cloud.github.com/assets/25094/1244989/d3218230-2a94-11e3-965d-134142b06567.png)

The core idea is to add a sub navigation menu when viewing a nested resource. This allows the user to keep context of where they are within the global navigation while being able to navigate between related nested resources.

This PR is to get feedback on the feature's overall idea and help define any additional DSL needs.

### Definition

The resources for the image above are defined as:

```ruby
ActiveAdmin.register Bid do
end

ActiveAdmin.regitster CostLine do
  belongs_to :bid
end

ActiveAdmin.register Resource do
  belongs_to :bid
end
```

### Tasks

- [ ] Considerations for backward compatibility with existing functionality for nested resource menus.
- [ ] Handle optional relationships (i.e. `belongs_to optional: true`).
- [ ] Better styles for sub menu.