TypeScript: StoreGroup#getState interface

From #68 

```diff
+    getState<T>(): T {
+        return (this._storeGroup as any).getState(); // TODO: remove casting `any`
```

> @saneyuki said
> I thought to introduce some new interfaces which is implemented by StoreGroup or Store to fix it.
> Perhaps is it not a right way?


We should resolve this TODO issue.