Fixed issue #369

[raphaelbarreto]

Tracker.update() now returns NULL if object is not found rather than always returning a Rect

[raphaelbarreto]

@justadudewhohacks, some CI tests failed, but none of them are related to the change.

[justadudewhohacks]

Thanks for the PR! Travis is complaining about: error: call of overloaded 'Set(NULL)' is ambiguous #define FF_RETURN(val) info.GetReturnValue().Set(val). I think it's better to return FF_RETURN(Nan::Undefined()) or FF_RETURN(Nan::Null()) instead of FF_RETURN(NULL).

[raphaelbarreto]

Done. I've replaced with FF_RETURN(Nan::Null())

[justadudewhohacks]

Thanks! Published your changes to npm.