Make 'media' an item level field.

Releases can consist of discs of various media types, most typically
CD/DVD combo releases. Having the media value an album level field
forces all discs to be of the same type. Making it an item level
field instead seems to be the proper thing to do here.

Unfortunately this also drops the ability to match releases by media
type in the autotagger.

I'm a bit unsure about the test modifications, comments welcome :)

Fixes #161