Move glue methods & BasicRendering to AbstractController

Next things that we were discussing with @josevalim on skype.

I've moved the `render_to_string`, `render_to_body` and `_normalize_render` method implementations to `AbstractController` as they were anything more as just a glue method invoking other methods.

Besides that I've moved `BasicRendering` from ActionPack to AbstractController and set `rendered_format` to text by default in AbstractController.