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.

In this case we're building a hash of everything below a transform so we can do high level instancing and what not. If you've got 10000 trees with different transforms but the same geometry hierarchy, you want 'em to all have the same hash, so you don't include the transform