Load the Klawr.UnrealEngine assembly when the ScriptPlugin starts up

Also fixed SafeArrayTypeMismatchException being thrown by
IKlawrAppDomainManager::SetNativeFunctionPointers(). I guess the
.NET Marshaler wasn't expecting a SafeArray of variants, or
maybe I just set it up wrong, in any case it works with a SafeArray of
64-bit integers.