Add "$ref" to meta-schema definitions.

But don't reference it yet as that involves some restructuring.

Author: handrews

schema.json

@@ -3,6 +3,16 @@
     "id": "http://json-schema.org/draft/schema",
     "title": "Core schema meta-schema",
     "definitions": {
+        "jsonReference": {
+            "type": "object",
+            "properties": {
+                "$ref": {
+                    "type": "string",
+                    "format": "uriref"
+                }
+            },
+            "required": [ "$ref" ]
+        },
         "schemaArray": {
             "type": "array",
             "minItems": 1,