20130329 - combined queries

[Dan Gravell]

This release allows multiple queries for Acoustid fingerprints, MB disc IDs and FreeDB disc IDs to be sent at once. The three queries are executed in order, returning the first batch of results found (e.g. if an Acoustid query finds results, the other queries are not executed).

Some smaller items:

- Content-Type is now application/json for release queries

- Cope with queries without artist name... shows an error message

Dan