Rename packets to messages

[carlbennett]

May warrant some discussion.

[Davnit]

I'm relatively indifferent to this. I still think of messages as "packets" and "packets" as payloads/fragments. I think a lot of older/existing code (outside of this project) refers to messages as packets as well... but the naming isn't really that important, IMO.

[nmbook]

I used "messages" when documenting BotNet on here, if you guys want an example. I think it's fine to leave it one way or the other.