Adjust HTTP server examples (part 2)

- Follow-up to #14 (in which I avoided to do a file rename so that the
diffs were clearer). This contains the file renaming (from
‘dead_simple’ to ‘hello_world’) to match what I tried to do in
[dartlang.org
PR#312](https://github.com/dart-lang/dartlang.org/pull/312).
- Updated README.md