This patch series tries to integrate all the feedback received in the
recent review from Eric Sunshine. It completely removes the confusing
changes to get_mw_namespace_id_for_page() because I believe they are
unrelated to the namespace support.

I also split up the last patch in 4 different patches for clarity and
fixed the vocabulary (it's "virtual" namespaces, not "special", which
is a specific namespace).

I left that die() in there because it makes the code a little cleaner
and I'm lazy.

Thanks again for the good feedback!