Implement XEP-0184 delivery notifications

This patch provides three components required to implement XEP-0184:

 * DeliveryReceiptRequest is a PacketExtension to request a receipt
 * DeliveryReceipt is a PacketExtension that contains the receipt
 * DeliveryReceiptManager to handle sending/receiving of requests and
   receipts.

Implementation:

For requesting a receipt, just add a new DeliveryReceiptRequest() to
your message or use the helper function:

    DeliveryReceiptManager.addDeliveryReceiptRequest(packet);

Register a ReceiptReceivedListener to find out if your packet arrived:

    DeliveryReceiptManager.getInstanceFor(myConnection)
	.registerReceiptReceivedListener(new ReceiptReceivedListener() {
	    @Override public void onReceiptReceived(String fromJid,
				    String toJid, String receiptId) {
		// receipt received for packet id receiptId
	    }

To answer a receipt request, you can either check incoming packets
manually:

    if (DeliveryReceiptManager.hasDeliveryReceiptRequest(packet)) {
	// send receipt
    }

Or you can enable automatic receipt transmission with:

    DeliveryReceiptManager.getInstanceFor(myConnection)
	    .setAutoReceiptsEnabled(true);

Signed-Off-By: Georg Lukas <georg@op-co.de>