Add support for displaying/retrieving by SKS hash to lookup and onak CLI

  Now we are storing the SKS hash details of a key add the ability to
  display the hash in /pks/lookup and retrieve it via the new hget
  function. This should be compatible with the way in which SKS extends
  lookup to support its hashes.

  Also add hget to the onak CLI tool and the -s option for showing the
  SKS hash of keys.