Add support for type references by name in avro-schemas

[opomuc]

{
  "name": "users",
  "type": "record",
  "fields": [
      {
        "name": "User_A",
        "type": {
	    "name": "user",
            "type": "record",
            "fields": [
              {"name": "username", "type": "string"},
              {"name": "phone", "type": "long"},
              {"name": "age", "type": "int"}
            ]
        }
      },
      {"name": "User_B", "type": "user"}
    ]
}