Output json when receiving messages from dbus and --json parameter is given

Fixes #138

src/main/java/org/asamk/signal/json/JsonReceiptMessage.java

@@ -0,0 +1,25 @@
+package org.asamk.signal.json;
+
+import org.whispersystems.signalservice.api.messages.SignalServiceReceiptMessage;
+
+import java.util.List;
+
+class JsonReceiptMessage {
+
+    long when;
+    boolean isDelivery;
+    boolean isRead;
+    List<Long> timestamps;
+
+    JsonReceiptMessage(SignalServiceReceiptMessage receiptMessage) {
+
+        this.when = receiptMessage.getWhen();
+        if (receiptMessage.isDeliveryReceipt()) {
+            this.isDelivery = true;
+        }
+        if (receiptMessage.isReadReceipt()) {
+            this.isRead = true;
+        }
+        this.timestamps = receiptMessage.getTimestamps();
+    }
+}