Adding hash function to SceneInterface

I've added a hash function with an enum that computes different kinds of hashes (for Object, Transform, child names, etc) for a given point in time.

This will allow better use of caching mechanisms when loading scenes as well as use of instances in engines such as Gaffer.

Is there any reason we can't use the filename (maybe plus modification time?) rather than this pointer? It would seem far more preferable to have a stable hash if we can - or are you just taking a shortcut now because you anticipate being able to use an underlying hash provided by IndexedIO very soon?

`MurmurHash::append( size_t )` isn't actually defined, and and in order for this to compile on OS C, you'd need to cast to `uint64_t`.