Make the JSON Schema compiler handle enums declared in other namespaces.

tools/json_schema_compiler/test/simple_api.json

 [
   {
     "namespace": "simple_api",
     "description": "This is a simple API.",
     "types": [
       {
         "id": "TestType",
         "type": "object",
         "properties": {
           "string": {
             "type": "string",
             "description": "Some string."
           },
           "boolean": {
             "type": "boolean",
             "description": "Some boolean."
           },
           "number": {
             "type": "number",
             "description": "Some double."
           },
           "integer": {
             "type": "integer",
             "description": "Some integer."
           }
         }
+      },
+      {
+        "id": "TestEnum",
+        "type": "string",
+        "enum": ["one", "two", "three"]
       }
     ],
     "functions": [
       {
         "name": "incrementInteger",
         "type": "function",
         "description": "Increments the given integer.",
         "parameters": [
           {
             "name": "num",
@@ skipping 117 matching lines @@
         "parameters": [
           {
             "name": "someTestType",
             "$ref": "TestType"
           }
         ]
       }
     ]
   }
 ]