add support for IGMP packets
IGMP3 aux_len should be zero'd
Merge branch 'adferguson-igmp_support'

Change hardcoded proto 2 (IGMP) -> 12 (Xerox-PUP) in Packet_Arbitrary since we now parse proto 2 packets.

Conflicts:
	lib/Packet.ml
	lib/Packet.mli