js: implement import/delete Key, some fixes

• Keyring.js
  • implemented importKey: importing one or more armored public key blocks.
  • implemented deleteKey: deleting a public Key from gpg.

• Key.js renamed property Key.armor to Key.armored

• Helpers.js: toKeyIDArray does not complain anymore if there are no keys. Not having Keys in e.g. signing keys in encrypt is legitimate and common, the complaints were getting spammy

• Errors.js: gpgme_errors now always pass an optional additional message, for easier debugging in minified code

• Connection.js: Fix in gpgme-json responses containing objects

• eslintrc.json: Start using eslint. A cleanup to conform to it is not done yet

• Added further tests for the new functionality