Make bibref query case-insensitive

See commit messages for more details.

This addresses tobie/specref#22
Here's what doing the above does:

 ```
1) Reference [[CURIE-20120607]] is maintained.
2) Reference [[CURIE-20130625]] is maintained.
3) Reference [[CURIE-20130822]] is maintained.
4) Reference [[html-20121025]] is maintained.
5) Reference [[html-20121217]] is maintained.
6) Reference [[html-20130806]] is maintained.
7) Reference [[html-20140204]] is maintained.
8) Reference [[html-20140429]] is maintained.
9) Reference [[html-20140617]] is maintained.
```