updates to fully hypermedia
Changes to be committed:
modified:   app/controllers/api/v1/category_controller.rb
modified:   app/controllers/api/v1/graph_controller.rb
modified:   app/models/category.rb
modified:   app/serializers/category_serializer.rb
modified:   app/serializers/link_serializer.rb
modified:   config/database.yml
modified:   test/controllers/api/v1/category_controller_test.rb
modified:   test/controllers/api/v1/graph_controller_test.rb