Make bibref query case-insensitive

See commit messages for more details.

This addresses tobie/specref#22
Catching those is relatively easy using the search tool.

Here's what you get for "html":

```json
{
    "html-20121025": {
        "aliasOf": "html5-20121025"
    },
    "html-20121217": {
        "aliasOf": "html5-20121217"
    },
    "html-20130806": {
        "aliasOf": "html5-20130806"
    },
    "html-20140204": {
        "aliasOf": "html5-20140204"
    },
    "html-20140429": {
        "aliasOf": "html5-20140429"
    },
    "html-20140617": {
        "aliasOf": "html5-20140617"
    }
}
```