Add a new constructor for making Model instances on non-android systems

Hello there,
thanks for Active Android, we started using it for our new Android application. I have a proposal though, because I need to construct some of my Model instances in a web service outside of Android and then JSON-serialize them and send them over to an Android phone to be saved/updated/deleted. Take a look at the new constructor that I wrote to solve my problem. I don't want to leave it like this, because we want to easily upgrade to newer ActiveAndroid versions when available.
Feel free to propose a different solution if this is not an acceptable one.

Many thanks
Igor