This implements the signed envelopes and routing records described in libp2p/specs#217.

I've also updated the peerstore implementation to fit the new API & will open a PR there in a sec.

Travis correctly marks this as a breaking API change, since the new methods on the peerstore make existing implementations no longer fit the interface. I think this means I need to bump the minor version?

I still feel like a Go newbie, so don't be shy about review feedback :)

Some TODO items:

• filter out local addresses when making a routing record for a Host
• ensure envelope type hint has valid multicodec prefix (right now it accepts any byte string)
  • maybe? we'd have to keep this updated as new multicodecs are added