Create single schema string by deferencing $ref in glTF schema JSON for embedding JSON scheme string in C++.

## Setup

Use python2

```
$ pip2 install jsonpath-rw
$ pip2 install simplejson
```

Put `ref_resolver.py` and `generate_single_schema_doc.py` to glTF schema direcotry(i.e. `$glTF/specification/2.0/schema`)

## Generate

Run `generate_single_schema_doc.py`

## TODO

* [ ] Print date and git commit id?