Utils: Implement IpChecksum helper class

Packets: Implemnet generic assignment of boost::optional's to parsers
Packets: Fix <n>_offset field definition in BOOST_PACKET_PARSER_DEFINE_FIELDS
Packets: Implement PacketTypeMixin::key() helper
Packets: Clarify documentation regiarding valid()/in-valid() packets
Packets/DefaultBundle: Implement EthernetPacket, EthVLanPacket and IpV4Packet's finalize()