Support for "media" property

[Section 4.3](http://json-schema.org/latest/json-schema-hypermedia.html#anchor10) of the JSON Schema specification mentions the `media` property, which is used to give additional hints about the semantics of non-JSON data that is encoded as a JSON string. For example:

``` json
{
"type": "string",
"media": {
"binaryEncoding": "base64",
"type": "image/png"
}
}
```

I think an annotation of some sort might help perhaps. The serializer can check to see if an annotation is present on that property and if so, it can generate the `media` property.