encode message content with msgpack

this changes ssb to encode messages with msgpack. this will enable
https://github.com/dominictarr/secure-scuttlebutt/issues/21

making this a pull request because that is the open opensource way https://github.com/rvagg/node-levelup/blob/master/CONTRIBUTING.md

(todo, make another pr for that CONTRIBUTING.mb)

Okay gotta board a plane. Will get indexing links working on the flight...