Use HTTPS for Facebook link
Use the first 500 chars to preview planet posts

Also remove "id" and "name" from HTML tags in the preview to avoid
conflicts with the main post.
Mention that lablgtk2 is needed for the Gtk tutorial
Fix redirected and broken (#fragment) links

Check made with http://validator.w3.org/checklink?uri=http%3A%2F%2Focaml.org%2F&summary=on&hide_type=all&depth=4&check=Check
Merge branch 'master' of https://github.com/ocaml/ocaml.org