Update encoding.py

AFI/SAFI family is now added in the JSON message.

Example:
```Text
{
    "exabgp": "3.4.0",
    "time": 1402662620,
    "id": "producer.local_5216_192.168.33.12",
    "counter": 5,
    "type": "update",
    "header": "FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF003B02",
    "body": "0000001C4001010050020006020100000004400304C0A8210C80040400000000180A0A0A180A0B0A",
    "neighbor": {
        "ip": "192.168.33.12",
        "message": {
            "update": {
                "attribute": {
                    "origin": "igp",
                    "as-path": [
                        4
                    ],
                    "med": 0,
                    "atomic-aggregate": false
                },
                "announce": {
                    "ipv4 unicast": {
                        "192.168.33.12": {
                            "10.10.10.0/24": {},
                            "10.11.10.0/24": {}
                        }
                    }
                }
            }
        }
    }
}
```