RIO-31 feat(really-core): Implement Reference Field Type

- refactor CollectionActor to be FSM
- handle reference field on create request on CollectionActor
- fix test cases for CollectionActor
- add reference field
- implement CollectionActorTest to extend CollectionActor and handle test messages like `GetState`
- fix test cases for CollectionViewMaterializer, ModelLoader and GorillaEventCenter
Merge pull request #16 from SalmaKhater/feat-reference-field

RIO-31 feat(really-core): Implement Reference Field Type
RIO-54 feat(really-core): Handle Reference field on Update request

- refactor update request to add handling of reference field
- implement test cases to cover updating reference field
Merge pull request #22 from SalmaKhater/feat-reference-field

RIO-54 feat(really-core): Handle Reference field on Update request
feat(really-core): Implements Delete command in Collection Actor