Rename to Service

Add ModelService interface to merge all Services

cmd/budgeteer/main.go

@@ -26,7 +26,7 @@ func main() {
 	tv := &jwt.TokenVerifier{}
 
 	h := &http.Handler{
-		UserService:         us,
+		Service:             us,
 		TokenVerifier:       tv,
 		CredentialsVerifier: bv,
 	}