replaced function pointer with function object

[rschaten]

I'm working on a sensor network projekt, using your excellent Homie framework. I have different sensors (light, temperature), and some actors (led, relay). I modularized my code, so I have a class for every kind of device. And I'm unable to subscribe my HomieNodes to topics when I'm inside of a method.

The problem is, that HomieNode.subscribe() only takes a function pointer as a second parameter, which I'm unable to provide. I'd rather give a function object.

With this change I'm able to subscribe my method ledOnHandler() in one of these two ways:

ledNode.subscribe("on", [this](String value) { return ledOnHandler(value); });
ledNode.subscribe("on", std::bind(&DeviceLed::ledOnHandler, this, std::placeholders::_1));

I successfully tested that the old way of doing this still works:

ledNode.subscribe("on", ledOnHandler);

So I don't think that this change breaks anything.

To be honest, I didn't make this change myself. I had a friend helping me, who is far more used to write code in C++. To me, this looks even stranger than usual... ;-)

I'd be happy to learn that there is another way to register, so that this change is obsolete. Please tell me if you know something. Thanks in advance!

[marvinroger]

I knew this would be requested. Thanks for the PR. :)