GuidReference binary serialization

[Xroft666]

I would suggest getting that GuidReference class to be completly compatible with binary serialization.
At the moment, when trying to save a reference, one would get errors like these

SerializationException: Type 'UnityEditor.SceneAsset' in Assembly 'UnityEditor, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null' is not marked as serializable.

Marking that to non serialized obviously would render its drawer not as cool as it is :)
Same goes even for the "gameObject" property, it somehow wants to serialize even that.