Markdown pages

Some pages converted when possible. Renamed other pages to use the .php extension.

Removed twitter page because it had to be renamed to twitter.php, and the only contents of that file was an include for twitter.php, which seems to be out of tree.

The "whitelist" of pages in index.php was removed (it does parameter validation by regexp instead, using the about page as 404), to add a page you just have to create a .md or .php file.