Add update by example API

[greenlaw110]

Add an "update by example API" so that the problem described in http://stackoverflow.com/questions/10663633/in-play-morphia-how-do-i-manage-updates?rq=1 could be solved by:

static public void Update(JsonObject body){
   Gson gs = new GsonBuilder().create();
   MyModel m = gs.fromJson(body, MyModel.class);
   // this call erases "privatefield" from the document: m.save()
   m.update(); // this update existing data with new fields only
}