Consistently format the three "Hello, world" examples (docs and README)

Consistently format the three "Hello, world" examples (docs and README)

- encourage use of make_app() (makes testing easier)
- use fully-qualified module paths (painful in real-world code,
  but makes examples clearer)
- don't define a main() function that is never called: just
  use a boring old "__name__ == __main__" block, so the code
  can be copy + pasted + executed immediately
- just use a tuple when defining routes, not url(...): this is
  supposed to be a simple, minimalist example