Opportunity to select the protocol in a Address

I'm sorry if I'm doing something wrong. I'm newby on github

You do not use the method 'socket' with the ability to change 'family':

``` python
class ClientTransport(object):
 ...
    def connect(self):
        stream = IOStream(self._address.socket(), io_loop=self._session._loop._ioloop)
        socket = ClientSocket(stream, self, self._encodings)
        socket.connect();
```

I have not found to influence the choice of protocol (AF_INET, AF_INET6).
This can be a nuisance when the client connects to localhost or another address where ipv6 is used.
By default, 'Client' will be use ipv6, although it is likely the 'Server' listening ipv4.
Additional Information:
  http://askubuntu.com/questions/32298/prefer-a-ipv4-dns-lookups-before-aaaaipv6-lookups

And your commit:
```
commit 886b26e18a5aca504c3ac8379095defaf90d5598
Author: Ken Sato <ksato9700@gmail.com>
Date:   Mon Nov 28 23:36:17 2011 -0800

    Found I'm getting IPv6 address for localhost.
    
    How can I bind both IPv4 and v6 address in the server side?
```

Maybe you could provide the choice of protocol in 'Address' class?