Update to NAN 2.0 broke functionality

[jeisinger]

The update to NAN 2.0 changed the usage of nan's SetWeak to using WeakCallbackType::kParameter. The latter only triggers a callback after the object was collected. So taking a reference to that object doesn't work anymore.

I'm introducing kFinalizer in the v8 API so you can get the old behavior, but until then, you should consider going back to the (deprecated) API you used before.