CMS-2091 Create RelationshipType.ts and RelationshipTypeJson.ts

Added RelationshipType and RelationshiptTypeJson TS classes and
test for RelationshiptType. Added modifiedTime and createdTime
getters to RelationshipType java class.
Merge pull request #368 from enonic/CMS-2091

CMS-2091 Create RelationshipType.ts and RelationshipTypeJson.ts