Adjust HTTP server examples

- Make consistent use of cascade operator (missing from first two
examples).
- Change ‘dead-simple server’ qualifier to ‘Hello world server’, which
seems more appropriate and slightly less … coarse (note that I have not
renamed the file so as to make it easier to view the diff).
- Show how to set the content type in the Hello world server (since
this is commonly needed).
- Remove unused imports.
- Misc. cleanup.
Merge pull request #14 from chalin/patch-0306-http-server

Adjust HTTP server examples