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.

Sure. My intention is to have it reviewed before cortex 9 leaves the alpha state. The pull request's intention was to just break the binary compatibility and define the interface, so David can go on in his project.