js: Keyring listing keys

• implementing Keyring methods:
  • Keyring.getKeys: has an additional option that retrieves the armor and secret state once at the beginning. This is power hungry, but allows for Keys to be used directly (without querying gpgme-json each call)
  • permittedOperations.js: reflect recent changes in the native counterpart, adding more options
  • Key: adding two methods for retrieving the armored Key block and for finding out if the Key includes a secret subkey.