Express manufacturer in Schema.org output

[osma]

E.g. the example/test record ekumeeninen-00585 has the manufacturer "Saarijärven Offset" but this is not expressed in the current Schema.org output.

[osma]

It's not clear where to put this information. It should probably go in the PublicationEvent (see #28), but WorldCat doesn't include it (see e.g. http://www.worldcat.org/oclc/58349510 which is also printed by "Saarijärven Offset" but this information is not represented in the WorldCat Linked Data)