Don't support models that aren't properly namespaced.

When running the specs, I got the following exception:

````
Circular dependency detected while autoloading constant Cms::UnscopedPage
````

I investigated, and this is what I found:

* Issue #770 asks for unconventionally-named models to be autoloaded. For example, `Product` can be defined in `app/models/some_subfolder/product.rb`.
* 854240f5c792dcd8615d6b7b2104be53b037b6f0 attempted to fix this issue.
* I don't think 854240f5c792dcd8615d6b7b2104be53b037b6f0 actually changed anything. With the added `UnscopedPage` model in `app/models/cms/unscoped_page.rb`, all specs passed before and after the change. Also, I manually checked to see that the return value of `all_models` didn't change before and after the fix.
* The reason for the exception with Rails 4 is this change in the ActiveSupport code: https://github.com/rails/rails/commit/b33700f5580b4cd85379a1dc60fa341ac4d8deb2. Somehow, `Cms::UnscopedPage` is trying to be loaded twice, and on the second time ActiveSupport throws an exception believing there to be a circular dependency. Cms::UnscopedPage doesn't exist but the file where it would be defined does exist.
* I think it was purely by chance that unconventionally-named models were autoloaded. With the Rails 4 change, it doesn't work.

**Bottom Line** I don't see a reason to support unconventionally-named models, so I got rid of `UnscopedPage`. There's a reason for conventions. At the very least, this issue can be returned to once rails_admin is fully working with Rails 4.

According to my count the number of failing specs is down to 19 now.