Fix for orphaned EntityReferences

[djsmith]

This adds an EventListener that deletes security information (EntityReferences) when any entity tracked by Rhino.Security is deleted from the system.

I had to use some reflection code to get the entity security key without being able to use the generic method, which also catches and ignores exceptions from ServiceLocator.GetInstance(), but I couldn't think of any better way to get the job done.

Regards Dan