Implement packet versioning scheme.

[jchv]

Right now, there are 2 properties that select how the packet parser works:

• Whether or not the packet is a server packet or a client packet.
• Which service the packet is (login, game, message)

In addition to this, we need some way to select a version. Because we do not know anything about PangYa's server infrastructure, we should rely on client versions as the basis for protocol versioning.

[jchv]

Some preliminary work was done in #32, but we still need:

• Actual packets with versioning
• Good UI support