Update to 1.7.6+ (Protocol version 5) (Part 2)

* Did all the serverbound packets
 - Modified tab-complete plugin to function with the new packet stuff
* Tried making a quick offline-mode handshake and login thing
 - Worked in my head, but not in practice
 - Cannot for the life of me figure this out at 04:30 in the morning
 - Can someone else try to fix that stuff?
* Changed varint.unpack_varint to take a socket instead of a buffer
 - Varints are really only used for networking, is there a reason to use
a buffer?