API: Allow user to register function for downlink

[FokkeZB]

Receiving a response is not straight forward. You have to send to receive and sendBytes() only returns the length. Then you'll have to use ttn.downlinkPort and ttn.downlink to get the rest of the info.

The Wire library lets you register a function to be called when a message is received. This is a much more friendly API already, although the function still doesn't receive all info as arguments, which I think it should.

Current:

void loop() {
  // Send a byte
  byte buf[1];
  buf[0] = 20;
  int downlinkBytes = ttn.sendBytes(buf, 1);

  if (downlinkBytes > 0) {
    debugPrintLn("Received " + String(downlinkBytes) + " bytes on port " + String(ttn.downlinkPort) + ":")

    for (int i = 0; i < downlinkBytes; i++) {
      debugPrint(String(ttn.downlink[i]) + " ");
    }

    debugPrintLn();
  }

  delay(20000);
}

Proposed:

void setup() {
  // ..

  ttn.onMessage(receiveMessage);
}

void loop() {
  // send a byte to poll
  var poll = { x00 };
  ttn.sendByes(poll, 1);
}

void receiveEvent(const byte* buffer, int length, int port) {
    debugPrintLn("Received " + String(length) + " bytes on port " + String(port) + ":")

    for (int i = 0; i < downlinkBytes; i++) {
      debugPrint(String(buffer[i]) + " ");
    }

    debugPrintLn();
}