Fix bug in FreeDB disc ID calculation when using discs or disc images.
Overhaul DiscID interfaces for lookup services.

They're now more consistent and provide easier interfaces for
generating IDs from discs, lists of tracks and cuesheets.
A few unusual edge cases were fixed in the process.
Add documentation for the CD lookup services.
Minor metadata lookup bugfix.
Fix FreeDB disc ID calculation regression.
Add several lookup-specific unit tests.