glTF 0.8

@fabrobinet for your immediate review.

CC #241 
This description "meshes_node_id" and "trs_node_id" might be a bit confusing. I'd prefer to have a single `node_id`.
The fact that one node contains a `mesh` doesn't not specialize it to a specific type as a node can hold `meshes` + 1 `camera`. it is not one or another.
also can we declare `rotation` , `scale` and `translation` to be simply at the same level and exclusive with `matrix` at the same level that would be just `node_id` ?