cleanup
setting up evaluators
Wrap up values evaluator and expose valueType
https://github.com/KhronosGroup/glTF/issues/187
https://github.com/KhronosGroup/glTF/issues/258
https://github.com/KhronosGroup/glTF/issues/232
https://github.com/KhronosGroup/glTF/issues/85
https://github.com/KhronosGroup/glTF/issues/216
https://github.com/KhronosGroup/glTF/issues/240
https://github.com/KhronosGroup/glTF/issues/238
https://github.com/KhronosGroup/glTF/issues/188
https://github.com/KhronosGroup/glTF/issues/232