Make bibref query case-insensitive

See commit messages for more details.

This addresses tobie/specref#22
FWIW, http://www.w3.org/TR/html/ and http://www.w3.org/TR/html5/ point to the same resource. :)

But that's not what this is about. Consider those references as opaque strings kept for legacy purposes only.

Also note that given how Respec works, spec's URL are defined on the client-side for drafts, and it's unclear to me  whether or not google would catch those atm.