glTF models

Roadmap: #927 

Before merging, this needs:

* [ ] Final models and screenshot for the Sandcastle example
* [ ] Credits for the models used in the specs
* [ ] Updates to `DynamicScene/*` as needed by @mramato 

We're working on a few other things that don't need to hold this up:

* [ ] Tutorial
* [ ] Hosted service to convert COLLADA to glTF (including setting the up axis)

`gltfDefaults.js` lives in `ThirdParty` for now and will have a separate repo as we work on the schema.

**F.A.Q.** - No, we can't increase the scope of this pull request, and, yes, we are too busy to entertain bikeshedding.