Clone packets with copy-on-write semantics

Create a clone of a packet with minimal overhead by sharing all
buffers with the original packet.  Individual buffers are copied
before they are modified.
Add Murmur hash function

A simple framework for hash functions is provided by lib.hash.base.
Two variants of the "Murmur" class of hash functions is provided by
lib.hash.murmur.
Add a pure Lua implementation of a Bloom filter

lib.bloom_filter provides a generic Bloom filter based on the x64_128
variant of the Murmur hash function.
Add class method to check for multicast MAC address in lib.protocol.ethernet
Add apps to provide multi-port Ethernet switches

The abstract class lib.bridge.base provides a constructor for a
multi-port Ethernet switch with split-horizon semantics.  The subclass
lib.bridge.flooding implements a simple flooding bridge while
lib.bridge.learning avoids flooding through MAC learning via Bloom
filters.
Merge pull request #333 from alexandergall/learning-bridge

Learning bridge based on Bloom filter