Add MS NLB heartbeat ethertype, as well as a basic printer

The protocol is undocumented but Wireshark extracts some useful bits of
info from the packet, so it was used as a reference.
Add two MS NLB test captures

msnlb1.pcap contains two heartbeat packets from a single cluster (two
nodes). msnlb2.pcap is the same capture with a lower snaplen to exercice
the truncation code path.
Merge pull request #51 from orebokech/msnlb-printer

Add MS NLB heartbeat ethertype, a basic printer, and 2 test files