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.

tests/integrationtests/TestingProxy.h

@@ -43,6 +43,12 @@ public:
         registerProxy();
     }
 
+    TestingProxy(sdbus::IConnection& connection, std::string destination, std::string objectPath)
+        : ProxyInterfaces(connection, std::move(destination), std::move(objectPath))
+    {
+        registerProxy();
+    }
+
     ~TestingProxy()
     {
         unregisterProxy();