js: change in initialization ancd connection handling

• The Connection will now be started before an object is created, to better account for failures.
• index.js: now exposes an init(), which returns a Promise of configurable <GpgME | gpgmeGpgME_openPGPCompatibility> with an established connection.
• TODO: There is currently no way to recover from a "connection lost"
• Connection.js offers Connection.isConnected, which toggles on port closing.