Support for altcoins

This small patch makes it easier to build clients for other altcoins based on protocoin. Here is a sample Litecoin client based on the provided examples:

```python
import socket
from protocoin.clients import *

class LitecoinClient(BitcoinClient):
    coin = "litecoin"

    def handle_message_header(self, message_header, payload):
        print "Received message:", message_header.command

    def handle_send_message(self, message_header, message):
        print "Message sent:", message_header.command

def run_main():
    sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    sock.connect(("82.27.193.66", 9333))
    client = LitecoinClient(sock)
    client.handshake()
    client.loop()

run_main()
```

To add other altcoins we'd just need to add to the MAGIC_VALUES dict.