moved xml hints to separate element, added response, resource samples

Per discussion in workgroup, moved hints for format-specific example generation.  For example, XML format hints live inside a `xml` attribute in a model property:

```json
{
  "id": {
    "type": "integer",
    "format": "int64",
    "description": "the ID of the user",
    "readOnly": true,
    "xml": {
      "attribute": true,
      "namespaceURI": "http://example.org",
      "prefix": "ns1"
    }
  }
}
```