Remove requirement for RepositoryObserver to implement Identifiable

Hi Todd,

I noticed that RepositoryObserver takes a generic that needs to implement Identifiable. I don't see why that requirement is necessary, but let me know if I'm wrong. The reason it was an issue for me is that my entity doesn't have a String id.

I've tested this for my use-case by installing my modified repo-express artifact locally, creating a StateChangePublishingObserver that extends AbstractRepositoryObserver<T> where T is not Identifiable, and adding that observer to my DAL. Upon creation of my non-Identifiable entity, I see the subpub message in Fiddler and it looks good.