Fix #93: Get signals working for multiple proxies.

* Proxy::sdbus_signal_handler() needs to return 0 instead of 1 in
  order to allow multiple proxies listening to a signal all being
  triggered.
* Add test for emitting a signal to multiple proxies on same
  connection.

src/Proxy.cpp

@@ -238,7 +238,7 @@ int Proxy::sdbus_signal_handler(sd_bus_message *sdbusMessage, void *userData, sd
 
     callback(message);
 
-    return 1;
+    return 0;
 }
 
 }