Implemented update checking functionality

src/FilterEngine.cpp

 /*
  * This file is part of Adblock Plus <http://adblockplus.org/>,
  * Copyright (C) 2006-2013 Eyeo GmbH
  *
  * Adblock Plus is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 3 as
  * published by the Free Software Foundation.
  *
  * Adblock Plus is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 252 matching lines @@
 
   jsEngine->SetEventCallback(eventName, std::tr1::bind(&FilterEngine::UpdateCheckDone,
       this, eventName, callback, std::tr1::placeholders::_1));
 
   JsValuePtr func = jsEngine->Evaluate("API.forceUpdateCheck");
   JsValueList params;
   params.push_back(jsEngine->NewValue(eventName));
   func->Call(params);
 }
 
-void FilterEngine::UpdateCheckDone(std::string eventName, FilterEngine::UpdaterCallback callback, JsValueList& params)
+void FilterEngine::UpdateCheckDone(const std::string& eventName, FilterEngine::UpdaterCallback callback, JsValueList& params)
 {
   jsEngine->RemoveEventCallback(eventName);
 
   std::string error(params.size() >= 1 && !params[0]->IsNull() ? params[0]->AsString() : "");
   callback(error);
 }