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