Extend launch() docstring
Use addListeners() instead of listenTo()
Use None for empty buffer IDs

The old code used -1 for empty buffer IDs.  We now use None.
Whitespace fixes
Don't convert EthAddrs to raw for ofp_match

The canonical format for ethernet addresses in POX is EthAddrs.  We
shouldn't convert them to raw unnecessarily.
Use is_multicast instead of isMulticast()

The latter is deprecated.
Deal with removed EthAddr.toInt()

EthAddr.toInt() was removed, but riplpox uses the MAC-to-Mininet-name
relationship, at least for catching unexpected source/destinations.
(If that's all it uses them for, it's possible it should log them as
warnings instead of raising an exception...)