add signed node support

[mafintosh]

adds support for signing/verifying graph nodes using the following api

var log = hyperlog(db, {
  publicKey: myPublicKeyBuffer,
  sign: function (node, cb) {
    myCrypto.sign(node.key, mySecretKey, function (err, signature) {
      cb(err, signature)
    })
  },
  verify: function (node, cb) {
    if (!node.signature || !node.publicKey) return cb(null, false) // only allow signed nodes
    myCrypto.verify(node.key, node.signature, node.publicKey, function (err, valid) {
      cb(err, valid)
    })
  }
})

[mafintosh]

Maybe .publicKey should be renamed to .identity? That probably indicates better that this could also be a hash of a public key fx. @substack thoughts?

[mafintosh]

Maybe .publicKey should be renamed to .identity? That probably indicates better that this could also be a hash of a public key fx. @substack thoughts?

[mafintosh]

publicKey is now identity

[mafintosh]

publicKey is now identity