REST Relation CRUD Service

* Add Relation between two Objects of known type
  PUT source-type/source-id/relation/trget-type/target-id

* Remove Relation between two Objects of known type
  DELETE source-type/source-id/relation/trget-type/target-id

* Find Relations between Objects based on known types
  GET source-type/source-id/relation/target-type

Issue: #36