Added << operators for key/index access of ofxGenericValueStore.

This allows for convenient access to sub-nodes in
ofPtr< ofxGenericValueStore > objects.

``` C++
ofPtr< ofxGenericValueStore > parent;
parent.createFromJSON( " { children : { Thelma : { age : 50, children: { Fred : { age : 25 } } } } } " );

ofPtr< ofxGenericValueStore > grandChild = parent << "children" << "Thelma" << "children" << "Fred"; 
ofPtr< ofxGenericValueStore > fredAge = grandChild << "age";  // fredAge.asInt() == 25 
ofPtr< ofxGenericValueStore > oops = parent << "children" << "Scooby"; // *oops == null;
```