Expose low-level api

[acrisci]

A low-level api would allow clients to send and receive arbitrary messages on the bus. This api would be needed for implementing dbus-send or dbus-monitor with this library.

let serial = bus.serial();
let msg = {
  type: METHOD_CALL,
  serial: serial,
  interface: 'org.test.interface',
  destination: 'org.test.name',
  member: 'SomeMethod',
  signature: 's',
  body: 'hello world'
};

bus.sendMessage(msg);

bus.on('message', (msg) => {
  if (msg.replySerial === serial) {
    // handle reply
  }
});