init: new default project layout

This migrates the old project layout to a new one with all files
contained in one directory, `Rain.d`, with details as follows:

* `Rainfile.yml` -> `Rain.d/config.yml`
* `urls.txt` -> `Rain.d/root.URLfile`

This can hopefully make the project root less cluttered and manageable.
Also it seems to be a step forward from all those XXXfiles pioneered by the
Node.js and Ruby camps... Let's see if it pans out :D
router: resolve include paths relative to the including file if possible
examples/hello: update project layout
examples/hello: update WSGI stub
router: allow omitting the .URLfile extension in includes
examples/hello: use and document .URLfile extension omission
examples/cache: update project layout and WSGI stub
examples/github-webhook: update project layout and WSGI stub
examples/scss: update project layout and WSGI stub
examples/socketio-chat: update project layout and WSGI stub
router: lift out router class probing
router: normalize attribute processing, remove special case
examples/task: update project layout and WSGI stub