Revert "Temporary fix for Netdate ("AUg" instead of "Aug")."

With the latest release of ocamlnet, this is no longer necessary.
This reverts commit 18acc3b74f73ca7974ea0714c0bd2ac6f59bcf85.
Require netstring >= 3.7.3
Add make "preview" target that make local URLs relative.

This target is very desirable to test the rendering on ones own
machine before pushing the changes.
Do not use absolute links when unnecessary (partly changed).
Improve syntax highlighting.