Define Performer formally as a union of person and organization

Use union of sets (OWL-DL compliant) to describe the class Performer.  Roughly:
```
owl:Class Performer
  owl:unionOf rdf:parseType="Collection"
    owl:Class rdf:about=foaf:Person
    owl:Class rdf:about=foaf:Organization
```

For reference: http://www.semantic-web-journal.net/system/files/swj432.pdf

Depends on #21 