Support IPv6

[error10]

MySQL supports IPv6 with version 5.5, but pymysql has no support for IPv6 and fails with a strange error if you attempt to use an IPv6 address:

  File "connections.py", line 743, in _connect
    raise OperationalError(2003, "Can't connect to MySQL server on %r (%s)" % (self.host, e.args[0]))
pymysql.err.OperationalError: (2003, "Can't connect to MySQL server on '2001:db8:81:2c::2' (-9)")

I'm no Python expert but I think the problem is that an AF_INET6 socket should be opened when trying to connect to an IPv6 address.

[error10]

I've worked around this and added IPv6 support with the following change. The problem is that it raises the minimum Python requirement to version 2.6. To keep compatibility with 2.4, a whole lot of extra code would be needed. Though, I'm not really sure if anybody's even still using 2.4 anymore...

                 if DEBUG: print 'connected using unix_socket'
             else:
-                sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
-                t = sock.gettimeout()
-                sock.settimeout(self.connect_timeout)
-                sock.connect((self.host, self.port))
-                sock.settimeout(t)
+                sock = socket.create_connection((self.host, self.port), self.connect_timeout)
                 self.host_info = "socket %s:%d" % (self.host, self.port)
                 if DEBUG: print 'connected using socket'

[methane]

@error10 I've merged your patch. Thanks!