Fix to analyze IPv6 traffic

This fix provides a support for analyzing traces with IPv6 packets.

This is what I got for a trace with IPv6 traffic only:
$ captcp statistic youtube-20130422-03-filtered.pcap
I experienced the following exception:
Traceback (most recent call last):
  File "/usr/bin/captcp", line 4290, in <module>
    sys.exit(captcp.run())
  File "/usr/bin/captcp", line 4261, in run
    pcap_parser.run()
  File "/usr/bin/captcp", line 552, in run
    self.callback(dt, packet.data)
  File "/usr/bin/captcp", line 826, in internal_pre_process_packet
    self.cc.update(ts, packet)
  File "/usr/bin/captcp", line 2459, in update
    c = Connection(packet)
  File "/usr/bin/captcp", line 2264, in __init__
    TcpConn.__init__(self, packet)
  File "/usr/bin/captcp", line 2155, in __init__
    self.sip       = Converter.dpkt_addr_to_string(ip.src)
  File "/usr/bin/captcp", line 480, in dpkt_addr_to_string
    iaddr = int(struct.unpack('I', addr)[0])
struct.error: unpack requires a string argument of length 4