js: First testing and improvements

• Introduced Mocha/chai as testsuite. After development build 'npm test' should run the unit tests. Functionality exclusive to Browsers/WebExtensions cannot be run this way, so some other testing is still needed.
  • package.json: Added required development packages
  • .babelrc indirect configuration for mocha. ES6 transpiling needs some babel configuration, but mocha has no setting for it.
  • test/mocha.opts Vonfiguration for mocha runs

• Fixed errors:
  • Helpers.js toKeyIdArray; isLongId is now exported
  • Key.js Key constructor failed
  • Message.js will not throw an Error during construction, a new message is now created with createMessage, which can return an Error or a GPGME_Message object

• Tests:
  • test/Helpers: exports from Helpers.js, GPGME_Error handling
  • test/Message: first init test with bad parameters