Add to FirebaseDemo an example of retrieving a subtree instead of a leaf node.

We need examples of using the 

```
JsonObjectSubscript<const char*> operator[](const char* key);
JsonObjectSubscript<const String&> operator[](const String& key);
JsonVariant operator[](JsonObjectKey key) const;
```

 methods on FirebaseObject. 

i.e. if your database is 

/test/values/value1
/test/values/value2

and you fetch "test/values" show accessing "value1" and "value2".
