Added Support for IP Tunnels

I noticed that data contained in an IP tunnel wasn't being processed. I added the linkages between the protocols to make this happen. I've tested:

IPv4 -> IPv4
IPv4 -> IPv6
IPv6 -> IPv4

Here are a couple of publicly available captures with IP tunnels:
http://packetlife.net/captures/IP_in_IP.cap
http://packetlife.net/captures/IPv6_in_IP.cap