allow for $schema as an optional type

[delaneyj]

When wanting to use a generated schema as validation

{
    "$schema": "http://json-schema.org/draft-04/schema#",
    "$ref": "#/definitions/GeneratorOptions",
    "definitions": {
        "XXX": {
            "properties": {
             "$schema": {"type":"string"},
...
}

This would allow for use of the jsonschema in a document even when additoonalProperties is set to false.