ruby constant syntax is not supported as routing `:controller` option.

The current implementation only works correctly if you supply the `:controller` with directory notation (eg. `:controller => 'admin/posts'`).

The ruby constant notation (eg. `:controller => 'Admin::Posts`) leads to unexpected problems with `url_for`.

This patch prints a warning for every non supported `:controller` option. I also added documentation how to work with namespaced controllers. The warning links to that documentation in the rails guide.