js: Key handling stubs, Error handling, refactoring

• Error handling: introduced GPGMEJS_Error class that handles errors at a more centralized and consistent position
• src/Connection.js: The nativeMessaging port now opens per session instead of per message. Some methods were added that reflect this change
  • added methods disconnect() and reconnect()
  • added connection status query
• src/gpgmejs.js
  • stub for key deletion
  • error handling
  • high level API for changing connection status
• src/gpgmejs_openpgpjs.js
  • added stubs for Key/Keyring handling according to current state of discussion. It is still subject to change
• src/Helpers.js
  • toKeyIdArray creates an array of KeyIds, now accepting fingerprints, GPGMEJS_Key objects and openpgp Key objects.
• Key objects (src/Key.js) Querying information about a key directly from gnupg. Currently a stub, only the Key.fingerprint is functional.
• Keyring queries (src/Keyring.js): Listing and searching keys. Currently a stub.