Added support for "post packet events".

NetworkMarker now contains a list of post listeners that are invoked 
(in no particular order) when a packet has been serialize and sent
to a player, OR, when it has been enqueued for processing by the 
server.

This works for both 1.7.2+ (Netty) and 1.6.4 and earlier, though the
1.6.4 version has a good deal more overhead.