add validation rules for matches
refs #95
Only concepts have matches, not collections.
A match always consists of a type of match (eg. exactMatch) and a uri.
A skos:exactMatch is disjoint with skos:narrowMatch, skos:broadMatch, skos:relatedMatch. But a skos:closeMatch is not (closeMatch is the parent of exactMatch). For simplicity's sake: A SKOS concept can only have one match with a certain URI during editing. So, a Concept that has an relatedMatch with another Concept can't also have a broadMatch with that Concept, although strictly speaking SKOS seems to support that.